Update serializer.rst

Hello

I think it is not necessary to add a ClassMetadataFactory here because we are not about to use Class Metadata, Without that argument, the example works well too

Author: issamkhadiri1989

components/serializer.rst

@@ -1574,7 +1574,7 @@ context option::
         }
     }
 
-    $normalizer = new ObjectNormalizer($classMetadataFactory);
+    $normalizer = new ObjectNormalizer();
     $serializer = new Serializer([$normalizer]);
 
     $data = $serializer->denormalize(