[git] reflect branch rename from 'master' to 'main' (#6)

CONTRIBUTING.md

@@ -53,7 +53,7 @@ For each new feature, create a working branch:
 
 ```bash
 # Create a working branch for your new feature
-git branch --track <branch-name> origin/master
+git branch --track <branch-name> origin/main
 
 # Checkout the branch
 git checkout <branch-name>
@@ -75,24 +75,24 @@ This will open up a text editor where you can craft your commit message.
 
 Prior to submitting your pull request, it's good practice to clean up your branch and make it as simple as possible for the original repo's maintainer to test, accept, and merge your work.
 
-If any commits have been made to the upstream master branch, you should rebase your development branch so that merging it will be a simple fast-forward that won't require any conflict resolution work.
+If any commits have been made to the upstream main branch, you should rebase your development branch so that merging it will be a simple fast-forward that won't require any conflict resolution work.
 
 ```bash
-# Fetch upstream master and merge with your repo's master branch
-git checkout master
-git pull upstream master
+# Fetch upstream main and merge with your repo's main branch
+git checkout main
+git pull upstream main
 
 # If there were any new commits, rebase your development branch
 git checkout <branch-name>
-git rebase master
+git rebase main
 ```
 
 At this point, it might be useful to squash some of your smaller commits down into a small number of larger more cohesive commits. You can do this with an interactive rebase:
 
 ```bash
 # Rebase all commits on your development branch
 git checkout
-git rebase -i master
+git rebase -i main
 ```
 
 This will open up a text editor where you can specify which commits to squash.