minor #20457 [Serializer] Add missing argument (famoser)

This PR was merged into the 6.4 branch.

Discussion
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[Serializer] Add missing argument

In the example, the `$context` is constructed, but then forgot to be passed to the `serializer`.

This functionality is first documented in 6.4, hence I target this branch.

Commits
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d94be3f Add missing argument

serializer.rst

@@ -1818,7 +1818,7 @@ property. This can be used instead of
             },
         ],
     ];
-    $jsonContent = $serializer->serialize($person, 'json');
+    $jsonContent = $serializer->serialize($person, 'json', $context);
     // $jsonContent contains {"name":"cordoval","age":34,"createdAt":"2014-03-22T09:43:12-0500"}
 
 Advanced Deserialization