Clarify which host to add to configuration

doc/administration/geo/replication/docker_registry.md

@@ -53,7 +53,7 @@ We need to make Docker Registry send notification events to the
    registry['notifications'] = [
      {
        'name' => 'geo_event',
-       'url' => 'https://example.com/api/v4/container_registry_event/events',
+       'url' => 'https://<example.com>/api/v4/container_registry_event/events',
        'timeout' => '500ms',
        'threshold' => 5,
        'backoff' => '1s',
@@ -65,7 +65,8 @@ We need to make Docker Registry send notification events to the
    ```
 
    NOTE:
-   Replace `<replace_with_a_secret_token>` with a case sensitive alphanumeric string
+   Replace `<example.com>` with the `external_url` defined in your primary site's `/etc/gitlab/gitlab.rb` file, and
+   replace `<replace_with_a_secret_token>` with a case sensitive alphanumeric string
    that starts with a letter. You can generate one with `< /dev/urandom tr -dc _A-Z-a-z-0-9 | head -c 32 | sed "s/^[0-9]*//"; echo`
 
    NOTE: