Format comment to be more readable on website

Due to the font size used for code blocks making it require
horizontal scrolling, the comment is not immediately seen
until a full copy of the code block is pasted into another editor

Also changes the text slightly to make it more apparent
that both the host and the port value may need changes based on
what the primary's configuration carries.

doc/administration/geo/replication/docker_registry.md

@@ -109,11 +109,14 @@ For each application and Sidekiq node on the **secondary** site:
 
 1. Copy `/var/opt/gitlab/gitlab-rails/etc/gitlab-registry.key` from the **primary** to the node.
 
-1. Edit `/etc/gitlab/gitlab.rb`:
+1. Edit `/etc/gitlab/gitlab.rb` and add:
 
    ```ruby
    gitlab_rails['geo_registry_replication_enabled'] = true
-   gitlab_rails['geo_registry_replication_primary_api_url'] = 'https://primary.example.com:5050/' # Primary registry address, it will be used by the secondary node to directly communicate to primary registry
+   
+   # Primary registry's hostname and port, it will be used by
+   # the secondary node to directly communicate to primary registry
+   gitlab_rails['geo_registry_replication_primary_api_url'] = 'https://primary.example.com:5050/' 
    ```
 
 1. Reconfigure the node for the change to take effect: