docs: update CONTRIBUTING.md
Since we have been supporting multithreaded tests, so an update
to testing info in CONTRIBUTING.md was needed.
@@ -40,7 +40,7 @@ Execute the commands below to run the tests:
docker run --name scylla-ci -d scylladb/scylla
# Run all tests
-SCYLLA_URI="$(docker inspect --format='{{ .NetworkSettings.IPAddress }}' scylla-ci):19042" cargo test -- --test-threads=1
+SCYLLA_URI="$(docker inspect --format='{{ .NetworkSettings.IPAddress }}' scylla-ci):19042" cargo test
```
## Contributing to the book
