Error in AppDelegate.m since there was no return statement. (#1945)

Since the docs suggested we remove everything inside the `didFinishLaunchingWithOptions`, without the return statement there was an `Control reaches end of non-void function` error since the function expects a boolean return. After adding this, the error goes away.

docs/docs/installation-ios.md

 ```
 NSURL *jsCodeLocation = [[RCTBundleURLProvider sharedSettings] jsBundleURLForBundleRoot:@"index.ios" fallbackResource:nil];
 [ReactNativeNavigation bootstrap:jsCodeLocation launchOptions:launchOptions];
-```
+
+return YES;
+```