stream: fix off-by-factor-16 error in comment

The high watermark is capped at 8 MB, not 128 MB like the comment
in lib/_stream_readable.js said.

PR-URL: https://github.com/nodejs/node/pull/2479


Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>