Fix libgit2 website links

[strugee]

Note that there's also this line: https://github.com/libgit2/rugged/blob/762fd37e32c14707d017589a7dfa082dae7d9821/test/coverage/cover.rb#L70

But I couldn't figure out where this file was on the new libgit2 website, or if it was even there still. So I just left it alone.