Fix docs for name of GitHub signature request header

The documentation correctly refers to it as `X-Hub-Signature` in every place but one, where it uses `X-GitHub-Signature`. This PR corrects this use.

I've verified that `X-Hub-Signature` is the correct value, and that there is no `X-GitHub-Signature` header sent, in my own personal use, and by reviewing GitHub's [webhook code](https://github.com/github/github-services/blob/f3bb3dd780feb6318c42b2db064ed6d481b70a1f/lib/service/http_helper.rb#L76).

The inconsistency in request header naming is probably something to fix, but that's someone else's problem. :smile: