minor symfony#15420 Fix doctrine mapping validation type error (luxifer)

This PR was merged into the 2.3 branch.

Discussion
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Fix doctrine mapping validation type error

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

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.

Commits
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2c9b1db Fix doctrine mapping validation type error

Author: fabpot

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')
             ));
         }
     }