Move `master` branch to `main` about stockportfolio

[jennybc]

We're ready to rename master to main here (and elsewhere)!

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. Someone with admin permissions on this repo needs to do the renaming,
   as I do not have the power:

   usethis::git_default_branch_rename()

   If you wish, you can read the documentation for git_default_branch_rename().
   Or do the equivalent from the command line and in the browser (see the blog post).
3. All remaining contributors 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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