state the argument name/give an example

Suggestion from  skedwards88.

Co-authored-by: Sarah Edwards <skedwards88@github.com>

Author: FlashSheridan

content/actions/hosting-your-own-runners/configuring-the-self-hosted-runner-application-as-a-service.md

@@ -79,7 +79,11 @@ You can manage the runner service in the Windows **Services** application, or yo
    ```
 {% endmac %}
 
-The command takes an optional argument to install as a different user.
+The command takes an optional `user` argument to install the service as a different user.
+
+```shell
+./svc.sh install --user <em>USERNAME</em>
+```
 
 ## Starting the service