minor #7709 [DependencyInjection] Add missing use in the example (michaelperrin)

This PR was merged into the 2.7 branch.

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[DependencyInjection] Add missing use in the example

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a9e86d4 [DependencyInjection] Add missing use in the example

Author: xabbuh

service_container/definitions.rst

@@ -75,6 +75,7 @@ fetched from the container::
 
     use AppBundle\Config\DoctrineConfigManager;
     use Symfony\Component\DependencyInjection\Definition;
+    use Symfony\Component\DependencyInjection\Reference;
 
     $definition = new Definition(DoctrineConfigManager::class, array(
         new Reference('doctrine'), // a reference to another service