Updating links to master branch to point to main branch

[MikeRogers0]

Summary

As we recently changed the default branch from master to main we should update the links to the repo in the code also.

Right now those links won't break, but they're not going to be pointing to the most up-to-date code. So I went through updated the links pointing to https://github.com/rails/rails/blob/master & https://github.com/rails/rails/tree/master to use the main branch instead.

Other Information

Here is the command I ran on MacOS to find & replace the references:

$ grep -ilr 'rails/blob/master' * | xargs -I@ sed -i '' 's/rails\/blob\/master/rails\/blob\/main/g' @
$ grep -ilr 'rails/tree/master' * | xargs -I@ sed -i '' 's/rails\/tree\/master/rails\/tree\/main/g' @