minor #6256 Fixed code example (twifty)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #6256).

Discussion
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Fixed code example

The new subscriber must be called after the default listener. The default sets the locale as
````php
$locale = $request->attributes->get('_locale');
$request->setLocale($locale);
````
which overwrites any local set before it.

Commits
-------

50bca94 Fixed code example

Author: xabbuh

cookbook/session/locale_sticky_session.rst

@@ -54,8 +54,8 @@ how you determine the desired locale from the request::
         public static function getSubscribedEvents()
         {
             return array(
-                // must be registered before the default Locale listener
-                KernelEvents::REQUEST => array(array('onKernelRequest', 17)),
+                // must be registered after the default Locale listener
+                KernelEvents::REQUEST => array(array('onKernelRequest', 15)),
             );
         }
     }