Docs: Add missing single quote in WP_User_Query::prepare_query() DocBlock.

Follow-up to [38715], [52226].

Props kkmuffme, mukesh27.
Fixes #62714.

git-svn-id: https://develop.svn.wordpress.org/trunk@59542 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/class-wp-user-query.php

@@ -202,7 +202,7 @@ public static function fill_query_vars( $args ) {
 	 *                                                - 'login__in'
 	 *                                                - 'user_nicename' (or 'nicename')
 	 *                                                - 'nicename__in'
-	 *                                                - 'user_email (or 'email')
+	 *                                                - 'user_email' (or 'email')
 	 *                                                - 'user_url' (or 'url')
 	 *                                                - 'user_registered' (or 'registered')
 	 *                                                - 'post_count'