Docs: Correct `$attachment` parameter type for `wp_prepare_attachment_for_js` filter.

Props danielbachhuber.
Fixes #42449.
Built from https://develop.svn.wordpress.org/trunk@42345


git-svn-id: https://core.svn.wordpress.org/trunk@42174 1a063a9b-81f0-0310-95a4-ce76da25c4cd