Fix casing on CallbackBase in 2.4 porting guide (#37616)

docs/docsite/rst/porting_guides/porting_guide_2.4.rst

@@ -138,7 +138,7 @@ Users:
 Developers:
 
 * If your callback does not inherit from ``CallbackBase`` (directly or indirectly via another callback), it will still work, but issue a deprecation notice.
-  To avoid this and ensure it works in the future change it to inherit from ``callbackBase`` so it has the new options handling methods and properties.
+  To avoid this and ensure it works in the future change it to inherit from ``CallbackBase`` so it has the new options handling methods and properties.
   You can also implement the new options handling methods and properties but that won't automatically inherit changes added in the future.  You can look at ``CallbackBase`` itself and/or ``AnsiblePlugin`` for details.
 * Any callbacks inheriting from other callbacks might need to also be updated to contain the same documented options
   as the parent or the options won't be available.  This is noted in the developer guide.