Fix SonataAdminBundle:CRUDController compatibility

Commit 56eb4150adc0cfac26d43eccbb41d3ea6975686e of @rande broken the MediaAdminController with this erorr:

```php
  [ErrorException]                                                                                                                                                                                           
  Runtime Notice: Declaration of Sonata\MediaBundle\Controller\MediaAdminController::showAction() should be compatible with that of Sonata\AdminBundle\Controller\CRUDController::showAction() in /Users/sa  
  muel/git/srozeio/website/vendor/sonata-project/media-bundle/Sonata/MediaBundle/Controller/MediaAdminController.php line 118                                                                                
```

Fix it.