Wrong parameter in sonata.media.admin.media.manager.

The commit will fix the following error:
[ErrorException] Catchable Fatal Error: Argument 1 passed to Sonata\DoctrineMongoDBAdminBundle\Model\ModelManager::__construct() must be an instance of Symfony\Bridge\Doctrine\ManagerRegistry, instance of Doctrine\ODM\MongoDB\DocumentManager given, called in ../vendor/sonata-project/media-bundle/Admin/Manager/DoctrineMongoDBManager.php on line 22 and defined in /Users/interrobang/Sites/aag/vendor/sonata-project/doctrine-mongodb-admin-bundle/Sonata/DoctrineMongoDBAdminBundle/Model/ModelManager.php line 39