Update migration guide to specify that `WKPort` has been removed

The [migration guide](https://github.com/ionic-team/cordova-plugin-ionic-webview/#migrating-to-5x) doesn't mention that the [`WKPort`](https://github.com/ionic-team/cordova-plugin-ionic-webview/blob/2.x/README.md#wkport) option has been removed when upgrading from v2.x. This tripped me up as described in #562 so I think the migration guide should be updated to reflect this. I know there is a statement that says the default origin in the Android WebView is `http://localhost`, but an explicit statement about `WKPort` would be clearer.