Small fix for the contributing link

on the main README.md page

Author: kewtree1408

README.md

@@ -20,4 +20,4 @@ Current tutorials are:
 ## Contributing
 
 These tutorials are maintained by [DjangoGirls](http://djangogirls.org/). If you find any mistakes or want to update the tutorial 
-please [follow the contributing guidelines](https://github.com/DjangoGirls/tutorial-extension/contributing/).
+please [follow the contributing guidelines](./contributing/README.md).