message: doc: update instructions for running benchmarks
diff: @@ -303,13 +303,13 @@ library benchmarks (especially RE2).
  If you're hacking on one of the matching engines and just want to see
  benchmarks, then all you need to run is:
  - $ ./bench/run rust
  + $ (cd bench && ./run rust)
  If you want to compare your results with older benchmarks, then try:
  - $ ./bench/run rust | tee old
  + $ (cd bench && ./run rust | tee old)
  $ ... make it faster
  - $ ./bench/run rust | tee new
  + $ (cd bench && ./run rust | tee new)
  $ cargo benchcmp old new --improvements
  The `cargo-benchcmp` utility is available here:
  