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The documentation seemed out-of-date.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