chore: Translate pressbooks-multi-institution.pot in zh

82% of minimum 25% translated source file: 'pressbooks-multi-institution.pot'
on 'zh'.

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

languages/pressbooks-multi-institution-zh.po

@@ -345,11 +345,11 @@ msgstr "输入用户名或电子邮件以查找现有用户。最多3人。"
 
 #: resources/views/institutions/form.blade.php:237
 msgid "Premium Member Buy-in"
-msgstr ""
+msgstr "收费用户买入"
 
 #: resources/views/institutions/form.blade.php:257
 msgid "Book Limit"
-msgstr ""
+msgstr "书籍限制"
 
 #: resources/views/institutions/form.blade.php:261
 msgid "For an unlimited institution, enter 0."