minor #4780 Add missing semicolon (NightFox7)

This PR was submitted for the 2.6 branch but it was merged into the 2.3 branch instead (closes #4780).

Discussion
----------

Add missing semicolon

Add missing semicolon to
```
$callback = function ($dateTime) {
        return $dateTime instanceof \DateTime
            ? $dateTime->format(\DateTime::ISO8601)
            : '';
    };
```

Commits
-------

9f3ba1c Add missing semicolon

components/serializer.rst

@@ -198,7 +198,7 @@ When serializing, you can set a callback to format a specific object property::
         return $dateTime instanceof \DateTime
             ? $dateTime->format(\DateTime::ISO8601)
             : '';
-    }
+    };
 
     $normalizer->setCallbacks(array('createdAt' => $callback));