Update running-on-device.md (#1312)

`NSAllowsArbitraryLoads` was introduced in facebook/react-native#19643 and defaults to `true`. That means that removing `localhost` from `NSExceptionDomains` is not enough anymore, as describe in #1058.

Kudos to @Ashoat who tracked down the origin of the issue: #1058 (comment)

Closes #1058

docs/running-on-device.md

@@ -270,7 +270,7 @@ You have built a great app using React Native, and you are now itching to releas
 
 App Transport Security is a security feature introduced in iOS 9 that rejects all HTTP requests that are not sent over HTTPS. This can result in HTTP traffic being blocked, including the developer React Native server. ATS is disabled for `localhost` by default in React Native projects in order to make development easier.
 
-You should re-enable ATS prior to building your app for production by removing the `localhost` entry from the `NSExceptionDomains` dictionary in your `Info.plist` file in the `ios/` folder. You can also re-enable ATS from within Xcode by opening your target properties under the Info pane and editing the App Transport Security Settings entry.
+You should re-enable ATS prior to building your app for production by removing the `localhost` entry from the `NSExceptionDomains` dictionary and setting `NSAllowsArbitraryLoads` to `false` in your `Info.plist` file in the `ios/` folder. You can also re-enable ATS from within Xcode by opening your target properties under the Info pane and editing the App Transport Security Settings entry.
 
 > If your application needs to access HTTP resources on production, see [this post](http://ste.vn/2015/06/10/configuring-app-transport-security-ios-9-osx-10-11/) to learn how to configure ATS on your project.