fix fork to organization

While working on our chat bot I came across a bug in forking to an organization - the body should be `{"organization":"orgName"}` not `{"org":"orgName"}` as it was before. http://developer.github.com/v3/repos/forks/#create-a-fork. It probably wasn't caught by tests because Github would happily just fork to the user and return a 202.

Happy to complete any additional work to get this fix in. 

Cheers