Update Installing.md (#3871)

Change `index.ios` to `index` in `AppDelegate.m`.

docs/docs/Installing.md

 
 	- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
 	{
-		NSURL *jsCodeLocation = [[RCTBundleURLProvider sharedSettings] jsBundleURLForBundleRoot:@"index.ios" fallbackResource:nil];
+		NSURL *jsCodeLocation = [[RCTBundleURLProvider sharedSettings] jsBundleURLForBundleRoot:@"index" fallbackResource:nil];
 		[ReactNativeNavigation bootstrap:jsCodeLocation launchOptions:launchOptions];
 		
 		return YES;