I18N: Remove `<code>` tags in `invalid_rest_prepare_callback` error message in...

I18N: Remove `<code>` tags in `invalid_rest_prepare_callback` error message in `register_theme_feature()` for consistency with other strings.

Follow-up to [48611].

See #50758.
Built from https://develop.svn.wordpress.org/trunk@48612


git-svn-id: https://core.svn.wordpress.org/trunk@48374 1a063a9b-81f0-0310-95a4-ce76da25c4cd