minor symfony#11887 Update serializer.rst (TakTaki)

This PR was merged into the 4.2 branch.

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Update serializer.rst

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6ea286d Update serializer.rst

Author: javiereguiluz

components/serializer.rst

@@ -639,7 +639,7 @@ When serializing, you can set a callback to format a specific object property::
     $encoder = new JsonEncoder();
 
     // all callback parameters are optional (you can omit the ones you don't use)
-    $dateCallback = function ($innerObject, $outerObject, string $attributeName, string $format = null, array $context = []) {
+    $callback = function ($innerObject, $outerObject, string $attributeName, string $format = null, array $context = []) {
         return $innerObject instanceof \DateTime ? $innerObject->format(\DateTime::ISO8601) : '';
     };