minor #13663 Update custom_normalizer.rst (zulkris)

This PR was merged into the 3.4 branch.

Discussion
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Update custom_normalizer.rst

$context is not supported by Symfony 3.4

Commits
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f6a1505 Update custom_normalizer.rst

Author: javiereguiluz

serializer/custom_normalizer.rst

@@ -47,7 +47,7 @@ to customize the normalized data. To do that, leverage the ``ObjectNormalizer``:
             return $data;
         }
 
-        public function supportsNormalization($data, $format = null, array $context = [])
+        public function supportsNormalization($data, $format = null)
         {
             return $data instanceof Topic;
         }