[Feedback]: README likely has incorrect syntax

[bmulholland]

What's on your mind?

https://github.com/OneSignal/onesignal-vue#advanced-usage

This section has a code example of OneSignal use directly, not via the vue methods. Given that addEventListener is in index.ts, I suspect it's intended to be:

**this.$**OneSignal.Notifications.addEventListener('click', (event) => {
  console.log("The notification was clicked!", event);
});