docs: remove rackt link (#fixes 55)

Author: gajus

README.md

@@ -4,7 +4,7 @@
 [![NPM version](http://img.shields.io/npm/v/redux-immutable.svg?style=flat-square)](https://www.npmjs.org/package/redux-immutable)
 [![Canonical Code Style](https://img.shields.io/badge/code%20style-canonical-blue.svg?style=flat-square)](https://github.com/gajus/canonical)
 
-`redux-immutable` is used to create an equivalent function of Redux [`combineReducers`](http://rackt.org/redux/docs/api/combineReducers.html) that works with [Immutable.js](https://facebook.github.io/immutable-js/) state.
+`redux-immutable` is used to create an equivalent function of Redux [`combineReducers`](http://redux.js.org/docs/api/combineReducers.html) that works with [Immutable.js](https://facebook.github.io/immutable-js/) state.
 
 When Redux [`createStore`](https://github.com/reactjs/redux/blob/master/docs/api/createStore.md) `reducer` is created using `redux-immutable` then `initialState` must be an instance of [`Immutable.Iterable`](https://facebook.github.io/immutable-js/docs/#/Iterable).