Update doc/benchmarking.md

Summary:
The documentation seemed out-of-date. Updated based on the output from
running the command: src/bench/bench_bitcoin

Test Plan:
run src/bench/bench_bitcoin and inspect output. check that documentation
matched test output.

Reviewers: Fabien, jasonbcox, deadalnix, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity, schancel

Maniphest Tasks: T59

Differential Revision: https://reviews.bitcoinabc.org/D2190