Add an example for the authorization token

doc/administration/geo/replication/docker_registry.md

@@ -51,12 +51,16 @@ We need to make Docker Registry send notification events to the
        'threshold' => 5,
        'backoff' => '1s',
        'headers' => {
-         'Authorization' => ['<replace_with_a_secret_token>'] # An alphanumeric string. Case sensitive and must start with a letter.
+         'Authorization' => ['<replace_with_a_secret_token>']
        }
      }
    ]
    ```
 
+   NOTE: **Note:**
+   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: **Note:**
    If you use an external Registry (not the one integrated with GitLab), you must add
    these settings to its configuration yourself. In this case, you will also have to specify