Add missing use statement to databases & doctrine page

Author: jontjs

doctrine.rst

@@ -813,6 +813,7 @@ with any PHP model::
 
     use App\Entity\Product;
     use App\Repository\ProductRepository;
+    use Doctrine\ORM\EntityManagerInterface;
     use Symfony\Component\HttpFoundation\Response;
     use Symfony\Component\Routing\Annotation\Route;
     // ...