Fix doctrine mapping validation type error

The error tell to implement the service `doctrine.orm.<em_name>.metadata_driver`, but this extensions check for the existence of the `doctrine.orm.<em_name>_metadata_driver` where `<em_name>` the entity manager name.

Author: luxifer

src/Symfony/Bridge/Doctrine/DependencyInjection/AbstractDoctrineExtension.php

@@ -252,7 +252,7 @@ protected function assertValidMappingConfiguration(array $mappingConfig, $object
             throw new \InvalidArgumentException(sprintf('Can only configure "xml", "yml", "annotation", "php" or '.
                 '"staticphp" through the DoctrineBundle. Use your own bundle to configure other metadata drivers. '.
                 'You can register them by adding a new driver to the '.
-                '"%s" service definition.', $this->getObjectManagerElementName($objectManagerName.'.metadata_driver')
+                '"%s" service definition.', $this->getObjectManagerElementName($objectManagerName.'_metadata_driver')
             ));
         }
     }