Dead link in Tutorials section

The tutorials section refers to a link to a French article that is no longer accessible:
https://www.demonixis.net/blog/category/tutoriels/tuto-xna/

This code from `articles/tutorials/index.html` should be removed:
```markdown
#### French

- [French articles about MonoGame on Linux, Windows and Windows 8](http://www.demonixis.net/blog/category/tutoriels/tuto-xna/)
```

