[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`)

Author: theStack

doc/benchmarking.md

@@ -11,7 +11,7 @@ Running
 For benchmarks purposes you only need to compile `bitcoin_bench`. Beware of configuring without `--enable-debug` as this would impact
 benchmarking by unlatching log printers and lock analysis.
 
-    make -C src bench_bitcoin
+    make -C src bitcoin_bench
 
 After compiling bitcoin-core, the benchmarks can be run with: