Fix websocket port option for serve

[marvinvi]

Websocket port option does not work since mdbook.rs use --websocket-port in arguments and use ws-port in implementation.

[azerupi]

Thank you for the PR!
So looking at it now, I would actually prefer to keep the more verbose name --websocket-port.
If you change that I can merge the PR :)

[marvinvi]

Done. :)

[azerupi]

Thank you!