Fix links in CONTRIBUTING.md

[brson]

Github is now helpfully displaying a link to CONTRIBUTING.md when opening an issue, but it doesn't look quite right: https://github.com/mozilla/rust/blob/master/CONTRIBUTING.md

The links need to be changed to http links instead of wiki links.

[catamorphism]

Yeah, I'd noticed that; I'd assumed wiki links would work, but I was wrong. Also, it's kind of odd that the link shows up when you open an issue, but not when you submit a pull request (though maybe there's nothing that can be done about that).

[catamorphism]

This is fixed.