benchstat: update readme
benchstat: update readme

The benchstat usage string used in the README file is out of date (it
was the old `benchstat -h` output).

This change updates the README with the `benchstat -h` output from the
latest benchstat.

Fixes golang/go#20727

Change-Id: Id87f61f088a03aa1c02719217af7a48b01fb173c
Reviewed-on: https://go-review.googlesource.com/84483
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>

golang/perf,golang/perf