Wrong docker-compose location at readthedocs.org

[C-Fu]

Current doc url: https://ralph-ng.readthedocs.io/en/stable/installation/installation/ Current description: Docker installation (experimental) You can find experimental docker-compose configuration in https://github.com/allegro/ralph/tree/ng/contrib directory. Be aware, it is still a beta.

Correct description: Docker installation (experimental) You can find experimental docker-compose configuration in https://github.com/allegro/ralph/tree/ng/docker directory. Be aware, it is still a beta.