Fix broken link in Vagrant Cloud API docs
Fix broken link in Vagrant Cloud API docs

This commit fixes the #TODO link with the proper docs link to upload a
provider.

chrisroberts/vagrant,mitchellh/vagrant,bryson/vagrant,chrisroberts/vagrant,bryson/vagrant,sni/vagrant,gitebra/vagrant,mitchellh/vagrant,marxarelli/vagrant,bryson/vagrant,sni/vagrant,bryson/vagrant,chrisroberts/vagrant,sni/vagrant,marxarelli/vagrant,gitebra/vagrant,marxarelli/vagrant,mitchellh/vagrant,gitebra/vagrant,gitebra/vagrant,sni/vagrant,mitchellh/vagrant,chrisroberts/vagrant,marxarelli/vagrant