chore: point towards the master branch for the CI badge

Reference: https://docs.github.com./en/actions/managing-workflow-runs/adding-a-workflow-status-badge#using-the-branch-parameter

Author: darrachequesne

README.md

@@ -1,7 +1,7 @@
 
 # Engine.IO: the realtime engine
 
-[![Build Status](https://github.com./socketio/engine.io/workflows/CI/badge.svg)](https://github.com./socketio/engine.io/actions)
+[![Build Status](https://github.com./socketio/engine.io/workflows/CI/badge.svg?branch=master))](https://github.com./socketio/engine.io/actions)
 [![NPM version](https://badge.fury.io/js/engine.io.svg)](http://badge.fury.io/js/engine.io)
 
 `Engine.IO` is the implementation of transport-based