Change build badge to point to master

Previously this was linked to all builds, so any failing builds on other branches would update the README's badge on master.

Author: dmadison

README.md

@@ -1,4 +1,5 @@
-# Adalight-FastLED ![Build Status](https://github.com/dmadison/Adalight-FastLED/workflows/build/badge.svg)
+# Adalight-FastLED
+[![Build Status](https://github.com/dmadison/Adalight-FastLED/workflows/build/badge.svg?branch=master)](https://github.com/dmadison/Adalight-FastLED/actions?query=workflow%3Abuild)
 
 ![Adalight-Rainbow](http://i.imgur.com/sHygxq9.jpg)