Change primary branch name from `master` to `main`

[wesleymatosdev]

As master term has slavery references, and we're an open source diverse community, it would be incredible to change the primary branch name to main, so we avoid offending descendants of enslaved people.

A few references that corroborate on why such a change worth our time:

• GitHub to replace "master" with an alternative term to avoid slavery references
• Why GitHub renamed its master branch to main

[ytmimi]

Thanks! I think this has come up before, but I can't find any references in the rustfmt repo right now. It's definitely not the team's intention to offend or make anyone feel unwelcome, and I think this is a change we'll make at some point.