Update CONTRIBUTING.md

Author: Morgan Rhodes

docs/CONTRIBUTING.md

@@ -19,12 +19,12 @@ top of things.
 ## Making Changes
 
 * Create a topic branch from where you want to base your work.
-  * This is usually the master branch.
+  * This is usually the main branch.
   * Only target release branches if you are certain your fix must be on that
     branch.
-  * To quickly create a topic branch based on master; `git checkout -b
-    fix/master/my_contribution master`. Please avoid working directly on the
-    `master` branch.
+  * To quickly create a topic branch based on main; `git checkout -b
+    fix/main/my_contribution main`. Please avoid working directly on the
+    `main` branch.
 * Make commits of logical units.
 * Check for unnecessary whitespace with `git diff --check` before committing.
 * Make sure your commit messages are in the proper format.