minor #10594 Fixes var name in callback (mauricesvay)

This PR was merged into the master branch.

Discussion
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Fixes var name in callback

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Commits
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48b1fec Fixes var name in callback

Author: javiereguiluz

components/serializer.rst

@@ -629,7 +629,7 @@ When serializing, you can set a callback to format a specific object property::
 
     // all callback parameters are optional (you can omit the ones you don't use)
     $callback = function ($innerObject, $outerObject, string $attributeName, string $format = null, array $context = array()) {
-        return $dateTime instanceof \DateTime ? $dateTime->format(\DateTime::ISO8601) : '';
+        return $innerObject instanceof \DateTime ? $innerObject->format(\DateTime::ISO8601) : '';
     };
 
     $normalizer->setCallbacks(array('createdAt' => $callback));