Rename repo default branch to main

[albertvillanova]

Rename repository default branch to main (instead of current master).

Once renamed, users will have to manually update their local repos:

• Upstream:

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

• Origin:
  Rename fork default branch as well at: https://github.com/USERNAME/lam/settings/branches
  Then:

  git fetch origin main
  git remote set-head origin -a
  

CC: @sgugger