Update "Fetch a subscriber" documentation so it shows that either "SUBSCRIBER_ID" or "EMAIL_ADDRESS" can be used as argument

[svenemtell]

The "Fetch a subscriber" API states that the argument should be a "SUBSCRIBER_ID", but "EMAIL_ADDRESS" works just as well, just like in the official documentation https://developers.mailerlite.com/docs/subscribers.html#fetch-a-subscriber.