Fix CONTRIBUTING once dataset scripts transferred to Hub
This PR updates the `CONTRIBUTING.md` guide, once the all dataset scripts have been removed from the GitHub repo and transferred to the HF Hub:
- #4974

See diff here: https://github.com/huggingface/datasets/commit/e3291ecff9e54f09fcee3f313f051a03fdc3d94b

Additionally, this PR fixes the line separator that by some previous mistake was CRLF instead of LF.