Add missing use statements in code sample (#4479)

Resources/doc/reference/getting_started.rst

@@ -232,6 +232,8 @@ and for ``admin.yml``:
 
     namespace AppBundle\DependencyInjection;
 
+    use Symfony\Component\HttpKernel\DependencyInjection\Extension;
+    use Symfony\Component\DependencyInjection\ContainerBuilder;
     use Symfony\Component\DependencyInjection\Loader;
     use Symfony\Component\Config\FileLocator;