The 'key' we're passing in to the Peer constructor is a PeerServer cloud API key. You can use ours for now, but you should sign up for your own free key.
However, the given link just leads to this page, which doesn't have instructions on how to get a PeerServer key.
So, How do I get a PeerServer Key?
The PeerJS documentation is old. You do not need a PeerServer key anymore (This change is permanent). Just omit the key option -
const peer = new Peer()