Add clientId example to JWT tutorial

[funkyboy]

Tutorial is here: https://www.ably.io/tutorials/jwt-authentication
It includes an example with capabilities

var jwtPayload =
    {
        'x-ably-capability': JSON.stringify({ '*': ['publish', 'subscribe'] })
    }

I'd add also an example with clientId, otherwise customers might have a hard time finding out the key name for clientId, which is x-ably-clientId

Related discussion: ably/ably-cocoa#664 (comment)