Fixed a bug with a user ID in generatePrivateChannelToken

[akniazev-forise]

Point me if I'm wrong but according to https://centrifugal.dev/docs/server/channel_token_auth#sub there should be a sub param instead of client to generate a proper private channel key for selected user ID. Without it getting invalid token response and also centrifugo checksubtoken -c config.json <TOKEN> shows "valid subscription token for anonymous user and channel "channel"".