Geo: Update docs to refer to gitlab_rails['geo_node_name']

Within the following pages:

• https://docs.gitlab.com/ee/user/admin_area/geo_nodes.html#common-settings
• https://docs.gitlab.com/ee/user/admin_area/geo_nodes.html#multiple-secondary-nodes-behind-a-load-balancer

we refer to the gitlab_rails[geo_node_name] setting, where as it really should be gitlab_rails['geo_node_name'] so it's a valid setting.