Add braces missed while adding docs for the `option_page_capability_{$option_page}` hook.

See [25372]. See #25229.

Built from https://develop.svn.wordpress.org/trunk@28833


git-svn-id: http://core.svn.wordpress.org/trunk@28637 1a063a9b-81f0-0310-95a4-ce76da25c4cd