docs: Fix contributing page URL.

Change-Id: I0bcb4fdbe4c0d4d512c0382187809c05defb6b18
Reviewed-by: Anthony Léonard <anthony.leonard@savoirfairelinux.com>

CONTRIBUTING.md

 # Contributing
 
 A guide on contributing to Ring can be found here:
- - https://ring.readthedocs.org/en/latest/contributing.html
+ - https://ring.readthedocs.io/en/latest/dev/contributing.html