Fix references to dotcloud repo.

Fixes running tests in clean environment.

Signed-off-by: Paul Nasrat <pnasrat@google.com>

Author: Paul Nasrat

CONTRIBUTING.md

@@ -8,6 +8,6 @@ the same rules and principles. If you're already familiar with the way
 Docker does things, you'll feel right at home.
 
 Otherwise, go read
-[Docker's contributions guidelines](https://github.com/dotcloud/docker/blob/master/CONTRIBUTING.md).
+[Docker's contributions guidelines](https://github.com/docker/docker/blob/master/CONTRIBUTING.md).
 
 Happy hacking!