bug #12180 Fix variable name (makmaoui)

This PR was merged into the 4.3 branch.

Discussion
----------

Fix variable name

<!--

If your pull request fixes a BUG, use the oldest maintained branch that contains
the bug (see https://symfony.com/roadmap for the list of maintained branches).

If your pull request documents a NEW FEATURE, use the same Symfony branch where
the feature was introduced (and `master` for features of unreleased versions).

-->

Commits
-------

fb57b76 Fix variable name

Author: wouterj

components/serializer.rst

@@ -655,7 +655,7 @@ 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)
-    $callback = function ($innerObject, $outerObject, string $attributeName, string $format = null, array $context = []) {
+    $dateCallback = function ($innerObject, $outerObject, string $attributeName, string $format = null, array $context = []) {
         return $innerObject instanceof \DateTime ? $innerObject->format(\DateTime::ISO8601) : '';
     };