[Editorial] Setting the value of chroma_sample_position

[xzhaao]

In 5.5.2 Color config syntax,

...
chroma_sample_position = CSP_UNKNOWN
...

In 6.4.2 Color config semantics, different options of chroma sample positions are defined, including: CSP_UNKNOWN, CSP_VERTICAL, CSP_COLOCATED and CSP_RESERVED, but none of the latter three is referenced in the spec text.

It is suggested to make the editorial cleanup for 5.5.2 Color config syntax as follows:

...
chroma_sample_position = 0
...

and remove the second column of the table for chroma_sample_position on Page 119 of av1-spec,