Rename branch

[mattesmohr]

This year GitHub changed the default branch name to main. Our branch name is still master.

I was wondering if we could change it too, since HTMKit is a part of the vapor-community. Tim and the others
where talking about in the Swift Forum.

What do you think? Does this change chain some issues?

[mattesmohr]

I have renamed the branch today, to go along with Github and Vapor.