doc: fix typo in stream doc

Fix small typo in Buffering section of stream doc.
PR-URL: https://github.com/nodejs/node/pull/7738


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>