Fix AppVeyor build badge.

Author: thomthom

.github/README.md

@@ -1,6 +1,6 @@
 # SketchUp Ruby API Stubs
 
-[![Gem Version](https://badge.fury.io/rb/sketchup-api-stubs.svg)](https://badge.fury.io/rb/sketchup-api-stubs) [![Build status](https://ci.appveyor.com/api/projects/status/6l5tqoawh44lut66?svg=true)](https://ci.appveyor.com/project/thomthom/rubocop-sketchup)
+[![Gem Version](https://badge.fury.io/rb/sketchup-api-stubs.svg)](https://badge.fury.io/rb/sketchup-api-stubs) [![Build status](https://ci.appveyor.com/api/projects/status/6l5tqoawh44lut66/branch/master?svg=true)](https://ci.appveyor.com/project/thomthom/ruby-api-stubs/branch/master)
 
 Auto-generated stubs for the SketchUp Ruby API. Useful for IDE intellisense and auto-complete.