doc: remove redundant only from doc/api/stream.md

PR-URL: https://github.com/nodejs/node/pull/14858


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>