doc: stdout/err/in are all Duplex streams

stdout, stderr and stdin are all Duplex streams but documentation
states otherwise

Fixes https://github.com/nodejs/node/issues/9201

PR-URL: https://github.com/nodejs/node/pull/11194


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>