Use correct ID for plugin search input label.

Make the search button name and ID consistent with the ones in WP_List_Table::search_box().

props joedolson.
fixes #28856.
Built from https://develop.svn.wordpress.org/trunk@29123


git-svn-id: http://core.svn.wordpress.org/trunk@28908 1a063a9b-81f0-0310-95a4-ce76da25c4cd