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

components/serializer.rst

@@ -1722,7 +1722,7 @@ context option::
  }
  }
 
- $normalizer = new ObjectNormalizer($classMetadataFactory);
+ $normalizer = new ObjectNormalizer();
  $serializer = new Serializer([$normalizer]);
 
  $data = $serializer->denormalize(