benchmark: update obsolete information pointer

A doc suggested in an error message is no longer the place to get the
information about required http benchmarkers. Update the error message
to point to the current location for the information.

PR-URL: https://github.com/nodejs/node/pull/12026


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>