doc: update benchmark CI test indicator in README

As of c7627da8, benchmark tests are run
in CI, but the README was not updated to indicate this.

PR-URL: https://github.com/nodejs/node/pull/35945


Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>