benchmark: update comment in common.js

Very minor update in benchmark/common.js
Not exactly a critical change, just continued cleaning
out of old joyent/node PRs that never landed.

Ref: https://github.com/nodejs/node-v0.x-archive/pull/8515
PR-URL: https://github.com/nodejs/node/pull/2399


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>