chore: rename 'master' references to 'main'

Author: JoshuaKGoldberg

README.md

@@ -2,7 +2,7 @@
 
 A set of examples to help demonstrate common configurations using Mocha. They should all run out-the-box.
 
-[![Build Status](https://github.com/mochajs/mocha-examples/workflows/Tests/badge.svg?branch=master)](https://github.com/mochajs/mocha-examples/actions?query=workflow%3ATests+branch%3Amaster) [![lerna](https://img.shields.io/badge/maintained%20with-lerna-cc00ff.svg)](https://lerna.js.org/)
+[![Build Status](https://github.com/mochajs/mocha-examples/workflows/Tests/badge.svg?branch=main)](https://github.com/mochajs/mocha-examples/actions?query=workflow%3ATests+branch%3Amain) [![lerna](https://img.shields.io/badge/maintained%20with-lerna-cc00ff.svg)](https://lerna.js.org/)
 
 ## Examples