Remove dead url reference.

[delath]

- What I did
I removed a sentence with a dead url.

- How I did it
By just deleting it, i would have preferred to fix the reference, but there doesn't seem to be a review process anymore in the official docs.

- How to verify it
Just by trying to access https://docs.docker.com/opensource/workflow/make-a-contribution/
I even searched for the old reference in the wayback machine but nothing similar is present in the current docs.

- Description for the changelog
Removed sentence containing invalid URL from contributing guidelines.

- A picture of a cute animal (not mandatory but encouraged)