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: nodejs#2479
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

lib/_stream_readable.js

@@ -190,7 +190,7 @@ Readable.prototype.setEncoding = function(enc) {
   return this;
 };
 
-// Don't raise the hwm > 128MB
+// Don't raise the hwm > 8MB
 const MAX_HWM = 0x800000;
 function roundUpToNextPowerOf2(n) {
   if (n >= MAX_HWM) {