doc: make stream.Readable consistent

Rest of the document uses `stream.Readable` or `stream.Writable`. This
patch makes the usage consistent in the parameter description.

PR-URL: https://github.com/nodejs/node/pull/16786


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>