CONTRIBUTING.md: replace coreutils by grep

[oech3]

No description provided.

[codspeed-hq]

Merging this PR will not alter performance

✅ 10 untouched benchmarks
⏩ 17 skipped benchmarks¹

---

_{Comparing oech3:patch-1 (f4c356d) with main (e8b93e9)}

Footnotes

1. 17 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[oech3]

We have more coreutils...

[lhecker]

Thanks!