doc: edit the benchmark guide

These are minor typographical and style improvements to the guide for
writing and running benchmarks.

PR-URL: https://github.com/nodejs/node/pull/12041


Reviewed-By: Nikolai Vavilov <vvnicholas@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>