service templates: clarify what "it" is

Co-authored-by: Steve Traylen <[email protected]>

README.md

@@ -59,7 +59,7 @@ systemd::unit_file { 'foo.service':
 ```
 
 If you're using "template" units (those with a `@` in it), you can
-simply enable it with a `service` resource. This, for example, will
+simply enable an instance of the template with a `service` resource. This, for example, will
 enable the `foo@bar` service unit, based on the `foo@` template:
 
 ```puppet