Fix file name in final "If you want more" final message

Another little PR, to fix the "extra credit" koan file name in the final "If you want more" message.

Before: 

> That was the last one, well done!
> 
> If you want more, take a look at **about_extra_credit_task.py**

After:

> That was the last one, well done!
> 
> If you want more, take a look at **about_extra_credit.py**