stream: change comment on duplex stream options

'options.XXX' did not convey to me how to use camelCase for different
readable and writable options on a duplex stream. Give an example
instead.

PR-URL: https://github.com/nodejs/node/pull/24247


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>