Fix the html upload

Rawgit is no more, alas, and the replacement we're using for the txt serves everything as text files so that's not useful for us.

We can probably pilfer mostly-setup gh-pages workflow stuff from the tlswg repos, that's where we got most of our other scripts in this repo from :P
See also https://github.com/tlswg/dtls13-spec/blob/master/.github/workflows/ghpages.yml