doing a release: this is about mat2, not mat

Additionally, fix a typo.

CONTRIBUTING.md

@@ -31,7 +31,7 @@ Since MAT2 is written in Python3, please conform as much as possible to the
 8. Push the tag with `git push --tags`
 9. Download the gitlab archive of the release
 10. Diff it against the local copy
-11. If there is no different, sign the archive with `gpg --armor --detach-sign mat-$VERSION.tar.xz`
+11. If there is no difference, sign the archive with `gpg --armor --detach-sign mat2-$VERSION.tar.xz`
 12. Upload the signature on Gitlab's [tag page](https://0xacab.org/jvoisin/mat2/tags) and add the changelog there
 13. Announce the release on the [mailing list](https://mailman.boum.org/listinfo/mat-dev)
 14. Sign'n'upload the new version on pypi with `python3 setup.py sdist bdist_wheel` then `twine upload -s dist/*`