Update index.md to mention rescript instead of reason (#25)

* Update index.md

Change reason -> rescript

* Update index.md

Author: MarcCoquand

docs/index.md

@@ -21,7 +21,7 @@ bindings for React Native features as components & APIs that are:
 
 It is often said that writing ReactJS code feels like "just using JavaScript".
 The same applies to _ReScript React_ with _ReScript React Native_: you will feel
-like you are "just using Reason".
+like you are "just using ReScript".
 
 ## How to use _ReScript React Native_ ?