benchmark: fix linting errors

4d78121b had lint errors that went
undetected. This fixes them.

PR-URL: https://github.com/nodejs/node/pull/5840


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>