main instead of master for mdn/yari

[peterbe]

Instructions now that main is the default branch:

git branch -m master main
git fetch origin
git branch -u origin/main main

origin here (in 2 places) refers to the mdn/yari repo, not your myusername/yari repo.So if you call it "upstream" type these instead:

git branch -m master main
git fetch upstream
git branch -u upstream/main main

[peterbe]

Here's the PR with the small left-over fixes necessary