Rename how-to primary branch to main

[derekeder]

Following https://github.com/datamade/how-to/pull/228, we should rename the primary branch for this repository from master to main

Doing so will require no code changes and GitHub will handle redirects as well as updating any pull requests. When this change is made, anyone with a local copy will need to run the following:

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

[derekeder]

This is complete.