Geo HA Documentation: Fix "Configure the tracking database" step

• Follow up from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8917#note_126572457:

@ashmckenzie I need to either open a follow up issue for this, or spend the time now testing it to figure out exactly what configuration is needed on an Omnibus-managed tracking DB machine.

It didn't make any sense at all to only link to the external database section. (This was my mistake)

• Configure a Geo HA tracking database to determine minimum configuration
• Fix/improve https://docs.gitlab.com/ee/administration/geo/replication/high_availability.html#step-3-configure-the-tracking-database-on-the-secondary-node