Fix broken iOS notification

Author: leemunroe

README.md

@@ -9,7 +9,7 @@
 
 [OneSignal](https://www.onesignal.com) is a free push notification service for mobile apps. This plugin makes it easy to integrate your native iOS app with OneSignal.
 
-![alt text](https://onesignal.com/images/ios_10_notification_image.gif)
+<p align="center"><img src="https://app.onesignal.com/images/ios_10_notification_image.gif" width="400" alt="iOS Notification"></p>
 
 #### Installation
 See OneSignal's [iOS Native SDK Setup Guide](https://documentation.onesignal.com/docs/ios-sdk-setup) for documentation.