Rename `master` branch to `main` on Syllabus repo

[40thieves]

Which module(s) and week(s) does this change affect? Module(s): N/A Week(s): N/A

What is the work that needs to be done?

We made an effort some time ago to rename branches master to main, but we never updated the Syllabus repo.

I recently discovered that Github now has functionality to automate the process, so I figured we should give that a shot: https://github.com/github/renaming#renaming-existing-branches. It even pops up a notice to contributors to update their local repos.

Why is this work important to do?

1. It's the Right Thing To Do™️
2. Consistency with our other repos

Who might need to know about this change?

@CodeYourFuture/global-syllabus

[40thieves]

Done.

There was a minor complication as there was an existing main branch, but it looks like it only had 1 commit different and those changes had already made it over to master.

Closing.