doc: use sentence case in benchmark doc

Apply sentence case consistently in headers in benchmark doc.

PR-URL: https://github.com/nodejs/node/pull/37351


Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>