Fixed broken link in CONTRIBUTING.md file.
Fixed broken link in CONTRIBUTING.md file.

This pull request fixes issue #11648

sni/vagrant,mitchellh/vagrant,mitchellh/vagrant,chrisroberts/vagrant,marxarelli/vagrant,chrisroberts/vagrant,chrisroberts/vagrant,marxarelli/vagrant,mitchellh/vagrant,sni/vagrant,sni/vagrant,marxarelli/vagrant,marxarelli/vagrant,mitchellh/vagrant,sni/vagrant,chrisroberts/vagrant