fixing xml example too.

Author: Damien Carrier

security.rst

@@ -1653,7 +1653,7 @@ and set the ``limiter`` option to its service ID:
                     <!-- 2nd argument is the limiter for username+IP -->
                     <srv:argument type="service" id="limiter.username_ip_login"/>
                     <!-- 3rd argument is the app secret -->
-                    <srv:argument type="service" id="%kernel.secret%"/>
+                    <srv:argument type="string">%kernel.secret%</srv:argument>
                 </srv:service>
             </srv:services>