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

I18N: Remove `<code>` tags in an error message in `WP_REST_Posts_Controller::get_item_schema()` for consistency with other strings.

Follow-up to [48610].

See #50759.
Built from https://develop.svn.wordpress.org/trunk@48613


git-svn-id: https://core.svn.wordpress.org/trunk@48375 1a063a9b-81f0-0310-95a4-ce76da25c4cd