Update getting_started.rst

We have an errors without this two additional strings.
These strings from Symfony Coockbook (http://symfony.com/doc/current/cookbook/bundles/extension.html)

Resources/doc/reference/getting_started.rst

@@ -208,6 +208,8 @@ For ``admin.xml`` use:
 
     namespace AppBundle\DependencyInjection;
 
+    use Symfony\Component\HttpKernel\DependencyInjection\Extension;
+    use Symfony\Component\DependencyInjection\ContainerBuilder;
     use Symfony\Component\DependencyInjection\Loader;
     use Symfony\Component\Config\FileLocator;