Added english language tag to metadata.yaml

[MilesConn]

This should be a fix for #18

To get this to work on send-to-kindle I used this site which had the following error message

Language C is not supported by Kindle. Documents may fail to convert. Continue or specify new language of the book in RFC 5646 format, e.g. en, fr, ja.

I specified en and then send-to-kindle worked. I've verified this works as intended using exiftool to compare the metadata before and after the patch.

Before the patch

ExifTool Version Number         : 12.76
File Name                       : graham.epub
Directory                       : .
File Size                       : 1622 kB
File Modification Date/Time     : 2024:05:30 11:32:51-04:00
File Access Date/Time           : 2024:05:30 11:32:50-04:00
File Inode Change Date/Time     : 2024:05:30 11:32:51-04:00
File Permissions                : -rw-r--r--
File Type                       : EPUB
File Type Extension             : epub
MIME Type                       : application/epub+zip
Identifier Id                   : epub-id-1
Identifier                      : urn:uuid:c00446c7-bd85-4be7-ad0e-b5a64653d857
Title Id                        : epub-title-1
Meta Refines                    : #epub-title-1
Meta Property                   : title-type
Meta                            : main
Title                           : A collection of Paul Graham writings
Date Id                         : epub-date
Date                            : 2024:05:30 15:32:48Z
Language                        : en-US
Creator Id                      : epub-creator-1
Creator                         : Paul Graham
Meta Scheme                     : marc:relators
Description                     : This an autogenerated ebook derived from the writing post of Paul Graham.
Rights                          : All writing are property of © Paul Graham. The cover image was taken from startupquote.com/post/868392835. The code to generate this ebook was made by Omar Olivares, drop star if you enjoy. https://github.com/ofou/graham-essays
Manifest Item Id                : ncx
Manifest Item Href              : toc.ncx
Manifest Item Media-type        : application/x-dtbncx+xml
Manifest Item Properties        : nav
Spine Toc                       : ncx
Spine Itemref Idref             : cover_xhtml
Spine Itemref Linear            : yes
Guide Reference Type            : toc
Guide Reference Title           : Essays
Guide Reference Href            : nav.xhtml

and after

ExifTool Version Number         : 12.76
File Name                       : graham.epub
Directory                       : .
File Size                       : 1621 kB
File Modification Date/Time     : 2024:05:30 11:30:57-04:00
File Access Date/Time           : 2024:05:30 11:30:56-04:00
File Inode Change Date/Time     : 2024:05:30 11:30:57-04:00
File Permissions                : -rw-r--r--
File Type                       : EPUB
File Type Extension             : epub
MIME Type                       : application/epub+zip
Identifier Id                   : epub-id-1
Identifier                      : urn:uuid:67215dcb-833d-46df-bdc2-6265dffefbd5
Title Id                        : epub-title-1
Meta Refines                    : #epub-title-1
Meta Property                   : title-type
Meta                            : main
Title                           : A collection of Paul Graham writings
Date Id                         : epub-date
Date                            : 2024:05:30 15:30:54Z
Language                        : en
Creator Id                      : epub-creator-1
Creator                         : Paul Graham
Meta Scheme                     : marc:relators
Description                     : This an autogenerated ebook derived from the writing post of Paul Graham.
Rights                          : All writing are property of © Paul Graham. The cover image was taken from startupquote.com/post/868392835. The code to generate this ebook was made by Omar Olivares, drop star if you enjoy. https://github.com/ofou/graham-essays
Manifest Item Id                : ncx
Manifest Item Href              : toc.ncx
Manifest Item Media-type        : application/x-dtbncx+xml
Manifest Item Properties        : nav
Spine Toc                       : ncx
Spine Itemref Idref             : cover_xhtml
Spine Itemref Linear            : yes
Guide Reference Type            : toc
Guide Reference Title           : Essays
Guide Reference Href            : nav.xhtml```

[ofou]

Thanks, at some point will try to match the quality of standardebooks with their tools