document how to add a dropin file with Hiera

admin-guide/configuration/services.md

@@ -95,8 +95,8 @@ base::services:
         options:
             Service:
                 ExecStart:
-                - ''
-                - '/usr/sbin/my_service --verbose'
+                    - ''
+                    - '/usr/sbin/my_service --verbose'
 ```
 
 If there are multiple dropins, you might also name them individually with the `dropin_name` parameter.