doc: don't use useless constructors in stream.md

PR-URL: https://github.com/nodejs/node/pull/13145
Refs: http://eslint.org/docs/rules/no-useless-constructor


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>