Update contributing docs to use cargo bench -p ruff_benchmark

[charliermarsh]

Summary

I found that cargo benchmark lexer didn't work as expected:

❯ cargo benchmark lexer
    Finished bench [optimized] target(s) in 0.08s
     Running benches/formatter.rs (target/release/deps/formatter-4e1d9bf9d3ba529d)
     Running benches/linter.rs (target/release/deps/linter-e449086ddfd8ad8c)

Turns out that cargo bench -p ruff_benchmark is now recommended over cargo benchmark, so updating the docs to reflect that.