Update SiteAdminController.php

Tested under Symfony 2.3:
[Wed Jun 19 23:09:42 2013] [error] [client 127.0.0.1] PHP Fatal error: Call to undefined method Symfony\Component\HttpFoundation\Session\Session::setFlash() in /home/.../vendor/sonata-project/page-bundle/Sonata/PageBundle/Controller/SiteAdminController.php on line 56, referer: http://localhost/admin/sonata/page/site/2/snapshots