Administration: Fix an index inconsistency in `get_admin_page_title()` function.

Ensure that `get_admin_page_title()` returns a value from pages registered using `add_menu_page()`.

Fixes #46081.

Props grapestain, valentinbora, SergeyBiryukov, audrasjb.


Built from https://develop.svn.wordpress.org/trunk@48500


git-svn-id: https://core.svn.wordpress.org/trunk@48262 1a063a9b-81f0-0310-95a4-ce76da25c4cd