Docs: Correct some @since MU notation that was broken in [41200].

Every function introduced by MU was introduced in 3.0.0 as this was when MU was merged.

See #41509


git-svn-id: https://develop.svn.wordpress.org/trunk@41714 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/user.php

@@ -692,7 +692,7 @@ function get_blogs_of_user( $user_id, $all = false ) {
 /**
  * Find out whether a user is a member of a given blog.
  *
- * @since MU (3.0.0) 1.1
+ * @since MU (3.0.0)
  *
  * @global wpdb $wpdb WordPress database abstraction object.
  *