doc: fix typo in streams.md

Slight grammar change to the readable event:`error`

PR-URL: https://github.com/nodejs/node/pull/12924


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>