Fixes bad link in docs

Summary:
Fixes link in `IntegrationWithExistingApps.md`.
Closes https://github.com/facebook/react-native/pull/15435

Differential Revision: D5600616

Pulled By: shergin

fbshipit-source-id: 64ff5c9e6956274adf2b832b2c71b5092eed26ae

docs/IntegrationWithExistingApps.md

@@ -809,7 +809,7 @@ Now just create a release build of your native app from within Android Studio as
 
 ### Now what?
 
-At this point you can continue developing your app as usual. Refer to our [debugging](/docs/debugging.html) and [deployment](docs/running-on-device.html) docs to learn more about working with React Native.
+At this point you can continue developing your app as usual. Refer to our [debugging](docs/debugging.html) and [deployment](docs/running-on-device.html) docs to learn more about working with React Native.
 
 <script>
 function displayTab(type, value) {