Fix documentation link in README.md

Summary:
Updated link for `integrating with existing apps` documentation
Closes #8842

Differential Revision: D3576490

fbshipit-source-id: 20d7c39dec615a3928ddc98ef31c8fbcf063f145

Author: awinograd

README.md

@@ -40,7 +40,7 @@ Please use these community resources for getting help. We use the GitHub issues
 ## Documentation
 
 [The website’s documentation](https://facebook.github.io/react-native/docs/) is divided into multiple sections.
-- There are **Guides** that discuss topics like [debugging](https://facebook.github.io/react-native/docs/debugging.html), [integrating with existing apps](https://facebook.github.io/react-native/docs/embedded-app-ios.html), and [the gesture responder system](https://facebook.github.io/react-native/docs/gesture-responder-system.html).
+- There are **Guides** that discuss topics like [debugging](https://facebook.github.io/react-native/docs/debugging.html), [integrating with existing apps](https://facebook.github.io/react-native/docs/integration-with-existing-apps.html), and [the gesture responder system](https://facebook.github.io/react-native/docs/gesture-responder-system.html).
 - The **Components** section covers React components such as [`View`](https://facebook.github.io/react-native/docs/view.html) and [`Navigator`](https://facebook.github.io/react-native/docs/navigator.html).
 - The **APIs** section covers other libraries like [Animated](https://facebook.github.io/react-native/docs/animated.html) and [StyleSheet](https://facebook.github.io/react-native/docs/stylesheet.html) that aren’t React components themselves.
 - Finally, React Native provides a small number of **Polyfills** that offer web-like APIs.