Correcting a file name

Author: Arthur-Milchior

CONTRIBUTING.md

@@ -2,6 +2,6 @@
 
 * [fork this project](https://github.com/gitpython-developers/GitPython/fork) on GitHub
 * For setting up the environment to run the self tests, look at `.travis.yml`.
-* Add yourself to AUTHORS.md and write your patch. **Write a test that fails unless your patch is present.**
+* Add yourself to AUTHORS and write your patch. **Write a test that fails unless your patch is present.**
 * Initiate a pull request