switch from master to main

CONTRIBUTING.md

@@ -7,7 +7,7 @@
     ```
     $ git remote add upstream [email protected]:rpearce/react-expanding-textarea.git
     $ git fetch upstream
-    $ git rebase upstream/master
+    $ git rebase upstream/main
     ```
 1. Check out a feature branch
     ```
@@ -19,11 +19,11 @@
     $ git push origin my-feature
     ```
 1. Create a [pull request](https://help.github.com/articles/about-pull-requests/)
-   from your branch to this repo's `master` branch
-1. When all is merged, pull down the upstream changes to your master
+   from your branch to this repo's `main` branch
+1. When all is merged, pull down the upstream changes to your main
     ```
     $ git fetch upstream
-    $ git merge upstream/master
+    $ git merge upstream/main
     ```
 1. Delete your feature branch (locally and then on GitHub)
     ```