webcam guide's assign/part-id does not work

[Michał Sawicz]

https://developer.ubuntu.com/en/snappy/guides/appliance-builder-guide-webcam/ says:

+ assign:
+ # note that the app-id can be from a different snap too, e.g.
+ # random-app_some-service
+ - part-id: webcam-demo

The 'part-id' suggests webcam-demo.* will work, but it does not. Just above that it mentions app-id, but supplying an actual app id to part-id: works.