Fix Docker file reference

As reported by Ming Fang, the Configuration Reference
section on docker was instructing users to update the
wrong configuration file. This patch fixes it to the
correct one

Cherry-Pick: https://review.openstack.org/#/c/52752/
Closes-Bug: 1241915

Change-Id: Ib425e7a1c0886fcef9373de0953a66dbded0cd6d
Backport: stable/havana
Merge "Fix Docker file reference" into stable/havana