Fix a query failing in UserSearch

@@ -91,7 +91,7 @@ class UserSearch implements ArrayAccess, Iterator, Countable {
             }
             }
         }
         }
 
 
-        mysqli_query("SET SESSION group_concat_max_len = 40960", $wpdb->dbh);
+        mysqli_query($wpdb->dbh, "SET SESSION group_concat_max_len = 40960");
 
 
         $query = "
         $query = "
           SELECT SQL_CALC_FOUND_ROWS
           SELECT SQL_CALC_FOUND_ROWS