doc: update pushStream docs to use err first

Refs: https://github.com/nodejs/node/pull/17406#issuecomment-356661798

PR-URL: https://github.com/nodejs/node/pull/18088


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>