Add space between required and 2fa-type

It's causing confusion for client implementation. See https://github.com/github/hub/pull/515#discussion_r10908721 for details.

Client implementations:
 * https://github.com/octokit/go-octokit/blob/24c866c6d831ace4b2753f2e939ab1aaeb265a55/octokit/error.go#L120

* https://github.com/octokit/octokit.rb/blob/ab0404e0f1c6d36ed52c6e9c8c9734a5f14ec839/lib/octokit/error.rb#L155