changed wording from master to main in the contributing.md file

[mturich]

Hey Kent,

I recently bought your testing js course and I was configuring my repo when I encountered the use of "master" as default branch in the description but the branches are already called main. (or is it just on my machine because I use main as default branch ?)

I simple propose to change the wording in the docs from master to main to remove that one pitfall while setting up the project.

[kentcdodds]

Thanks!