docs: update readme

Author: pemrouz

README.md

@@ -2,4 +2,4 @@
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 [![Build Status](https://travis-ci.org/rijs/singleton.svg)](https://travis-ci.org/rijs/singleton)
 
-Exposes the Ripple instance globally (`window`/`global`)
+Exposes the instance globally on `(window || global).ripple`.