Change from `master` to `main`

### Issue
`master` as a branch name has been deprecated for some time now as it is often perceived as offensive and reminiscent of times bygone. 

[Why rename](https://sfconservancy.org/news/2020/jun/23/gitbranchname/
) 

[GitHub's process on renaming](https://github.com/github/renaming)

### Solution
Rename `master` as `main`