Rename main branch

[hardbyte]

I've renamed in GitHub, this PR just updates the docs.

Users that already have the project checked out may have to run:

git branch -m master main
git fetch origin
git branch -u origin/main main
git remote set-head origin -a

[codecov[bot]]

Codecov Report

Merging #669 (76e93f6) into main (d606c9a) will not change coverage. The diff coverage is n/a.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #669 +/- ## ======================================= Coverage 94.42% 94.42% ======================================= Files 13 13 Lines 969 969 ======================================= Hits 915 915 Misses 54 54 ```