minor #16192 Update doctrine.rst (guillaumebdx)

This PR was merged into the 5.4 branch.

Discussion
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Update doctrine.rst

Since 5.4, documentation inject ManagerRegistry but kept Doctrine\ORM\EntityManagerInterface in uses.

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Commits
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d957600 Update doctrine.rst

Author: javiereguiluz

doctrine.rst

@@ -361,7 +361,7 @@ and save it::
 
     // ...
     use App\Entity\Product;
-    use Doctrine\ORM\EntityManagerInterface;
+    use Doctrine\Persistence\ManagerRegistry;
     use Symfony\Component\HttpFoundation\Response;
 
     class ProductController extends AbstractController