i18n: Add missing Arabic translation for 'Preview Only' in ar/message… (#12456)

Co-authored-by: Raymond Berger <RayBB@users.noreply.github.com>

Author: hichemferjani

openlibrary/i18n/ar/messages.po

@@ -6231,6 +6231,9 @@ msgstr "فقط"
 msgid "Preview Book"
 msgstr "معاينة الكتاب"
 
+#: BookPreview.html
+msgid "Preview Only"
+msgstr "معاينة فقط"
 #: DonateModal.html
 msgid ""
 "Donate this book to the <a href=\"https://archive.org\">Internet "