fixed strict violation in ACL plugin

lib/plugins/acl/action.php

@@ -20,7 +20,7 @@ class action_plugin_acl extends DokuWiki_Action_Plugin {
      * @param Doku_Event_Handler $controller DokuWiki's event controller object
      * @return void
      */
-    public function register(Doku_Event_Handler &$controller) {
+    public function register(Doku_Event_Handler $controller) {
 
         $controller->register_hook('AJAX_CALL_UNKNOWN', 'BEFORE', $this, 'handle_ajax_call_acl');