Fix a typo in the PHPDoc for `wp_register_sidebar_widget()`.

Props Jonnyauk.
Fixes #29007.

Built from https://develop.svn.wordpress.org/trunk@29276


git-svn-id: http://core.svn.wordpress.org/trunk@29058 1a063a9b-81f0-0310-95a4-ce76da25c4cd
After [29179], remove the last instance of `this.isCompatibleSrc()`.

Props kovshenin.
See #28905.


Built from https://develop.svn.wordpress.org/trunk@29277


git-svn-id: http://core.svn.wordpress.org/trunk@29059 1a063a9b-81f0-0310-95a4-ce76da25c4cd