doc: fix example in stream doc

Fix cases where `chunk` is a buffer.

PR-URL: https://github.com/nodejs/node/pull/8378
Fixes: https://github.com/nodejs/node/issues/8085


Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>