Remove reference to react-router-native

Summary:
We're currently taking ownership of react-router-native and have shipped several beta releases that do not wrap the NavigationExperimental API. In order to avoid confusion, I'd appreciate it if we could avoid mentioning react-router-native here. Thanks :)
Closes https://github.com/facebook/react-native/pull/12691

Differential Revision: D4654551

Pulled By: hramos

fbshipit-source-id: d1f393cefc87f087d4e9c19b7cbee861f0a2d29a

docs/Navigation.md

@@ -148,7 +148,7 @@ Check out the [`NavigatorIOS` reference docs](docs/navigatorios.html) to learn m
 
 Since early 2016, React Native has shipped with an experimental re-implementation of the original `Navigator` component called `CardStack`. The major benefit it had over `Navigator` is the smooth native-thread animations provided by the Animated library.
 
-Because `NavigationExperimental` only included view components, it required a lot of boilerplate to use by itself. Several libraries sprung up around it, making it easier to use. Libraries such as `react-native-router-flux`, `ex-navigation`, and `react-router-native` all wrapped NavigationExperimental views in an easier-to-use API. Authors of many of these libraries now support React Navigation.
+Because `NavigationExperimental` only included view components, it required a lot of boilerplate to use by itself. Several libraries sprung up around it, making it easier to use. Libraries such as `react-native-router-flux` and `ex-navigation` wrapped NavigationExperimental views in an easier-to-use API. Authors of many of these libraries now support React Navigation.
 
 The `CardStack` and other NavigationExperimental views live on as a part of the React Navigation project. The new library aims to be easy to use, while continuing to enable the smooth and customizable animations that NavigationExperimental pioneered.