Move `master` branch to `main` about archive

[jennybc]

I have moved the default branch from master to main here (and elsewhere)!

When I ran usethis::git_default_branch_rename(), I saw this output, which might require some follow-up:

✓ Default branch of the source repo 'r-lib/archive' has moved: 'master' --> 'main'
• Be sure to update files that refer to the default branch by name.
  Consider searching within your project for 'master'.
x Some badges may refer to the old default branch 'master':
  - 'README.Rmd'
✓ Default branch of local repo has moved: 'master' --> 'main'

As a result, I won't close this issue, but rest assured that the default branch has been renamed.

This blog post announces the larger effort and explains how contributors can update their local environments:

Renaming the default branch

Here's the TL;DR:

1. Update usethis (you want usethis v2.1.2 or higher):

   install.packages("usethis")

2. Anyone with an existing fork or clone can adjust to the renamed default branch with:

   usethis::git_default_branch_rediscover()

   If you wish, you can read the documentation for git_default_branch_rediscover().
   Or do the command line equivalent (see the blog post).

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