Changing the main branch name to main

[lolipopshock]

Hi LayoutParser developers!

We've renamed the main branch to main from master, and this means you might need to execute some changes to your local clone of the LayoutParser repo. Following the instruction here, you just need to run the following commands in your local LayoutParser folder:

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

git remote prune origin  # <-- Optional