Fix some deprecated directives in the COTNRIBUTING.md file

CONTRIBUTING.md

@@ -13,8 +13,8 @@ this will greatly improve the odds of your merge-request getting merged.
 # Doing a release
 
 1. Update the [changelog](https://0xacab.org/jvoisin/mat2/blob/master/CHANGELOG.md)
-2. Update the version in the [main.py](https://0xacab.org/jvoisin/mat2/blob/master/main.py) file
+2. Update the version in the [mat2](https://0xacab.org/jvoisin/mat2/blob/master/mat2) file
 3. Update the version in the [setup.py](https://0xacab.org/jvoisin/mat2/blob/master/setup.py) file
-4. Commit the changelog and the main.py file
+4. Commit the changelog, mat2 and setup.py files
 5. Create a tag with `git tag -s $VERSION`
 6. Push the tag with `git push --tags`