[doc] fix Makefile target in benchmarking.md
[doc] fix Makefile target in benchmarking.md

while the resulting binary is called `bench_bitcoin`, the Makefile target is
named `bitcoin_bench` (see `src/Makefile.bench.include`)


Former-commit-id: a8428d317406ae7337db772d497363ce33dce7c9
syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin