Updated link to Code of Conduct in CONTRIBUTING.md

It seems that the correct link is https://github.com/mozilla/rust/wiki/Note-development-policy#wiki-conduct now whereas it was https://github.com/mozilla/rust/wiki/Note-development-policy#conduct before.
auto merge of #1649 : mrshu/servo/patch-1, r=larsbergstrom

It seems that the correct link is https://github.com/mozilla/rust/wiki/Note-development-policy#wiki-conduct now whereas it was https://github.com/mozilla/rust/wiki/Note-development-policy#conduct before.