chore: Translate pressbooks-book.pot in zh

97% of minimum 25% translated source file: 'pressbooks-book.pot'
on 'zh'.

Sync of partially translated files: 
untranslated content is included with an empty translation 
or source language content depending on file format

Author: transifex-integration[bot]

languages/zh.po

@@ -614,11 +614,11 @@ msgstr "在书中搜索："
 #: searchform.php:4
 msgctxt "placeholder"
 msgid "Search in book …"
-msgstr ""
+msgstr "在书中搜索…"
 
 #: searchform.php:8
 msgid "Search"
-msgstr ""
+msgstr "搜索"
 
 #: single.php:57
 msgid "Share This Book"