bug symfony#11781 Update serializer.rst (Raulnet)

This PR was merged into the 4.3 branch.

Discussion
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Update serializer.rst

Fix name variable in code exemple.

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Commits
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cd23bde Update serializer.rst

Author: OskarStark

components/serializer.rst

@@ -661,7 +661,7 @@ When serializing, you can set a callback to format a specific object property::
 
     $defaultContext = [
         AbstractNormalizer::CALLBACKS => [
-            'createdAt' => $callback,
+            'createdAt' => $dateCallback,
         ],
     ];