add domain to the create command example

docs/installation_instructions.md

@@ -185,7 +185,7 @@ its *hostname* and its *IP address*. Also check that your VPS meets our
 Create the OpenAppStack settings for your VPS by running the following command: 
 
 ```
-$ python -m openappstack create --ip-address IP_ADDRESS --hostname HOSTNAME
+$ python -m openappstack create --ip-address IP_ADDRESS --hostname HOSTNAME --subdomain oas example.org
 ```
 
 ### DNS entries