chore: Translate pressbooks-book.pot in zh

99% 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

@@ -622,4 +622,4 @@ msgstr "搜索"
 
 #: single.php:57
 msgid "Share This Book"
-msgstr ""
+msgstr "分享该书"