OK - Simple Answer:
\xampp\apache\error\HTTP_FORBIDDEN.html.var is the template file that is used for that message.
You can make changes to that file to change the output text. You may also need to make changes to the 'include' files as well.
Just remember the message they receive is driven by their browser/OS language choice, so if you have 'global' traffic - you will need to make multiple changes.