[Security] Fixing PHP example for limiting login attempts with rate limiter

Author: Damien Carrier

security.rst

@@ -1697,7 +1697,7 @@ and set the ``limiter`` option to its service ID:
                     // 2nd argument is the limiter for username+IP
                     new Reference('limiter.username_ip_login'),
                     // 3rd argument is the app secret
-                    new Reference('kernel.secret'),
+                    param('kernel.secret'),
                 ]);
 
             $security->firewall('main')