Update README.md to reference the gh-pages branch

Author: mjwilson-google

README.md

@@ -2,7 +2,7 @@
 
 This repository contains the latest editor's draft of the [W3C AudioWG](https://www.w3.org/2011/audio/)'s Web MIDI API.
 
-You can preview the current version of the master branch [here](https://webaudio.github.io/web-midi-api/).
+You can preview the current version of the gh-pages branch [here](https://webaudio.github.io/web-midi-api/).
 
 # Tests