Default branch is now called master

[greenbreakfast]

@vsemi @mh1412 @VSemi-Zhen

For compatibility with the documentation hosting service, I renamed the default branch to master.

If you have an existing local copy of the repo, you'll need to switch to from the old main branch to the new master branch.
(Unless you have any local changes, easiest way to make the switch is to delete the existing local copy and check out a new one)