Fix typo in service example (docs)

[smheidrich]

The example from the docs on the high-level service interface currently fails with:

Traceback (most recent call last):
  File "example.py", line 65, in <module>
    asyncio.get_event_loop().run_until_complete(main())
  File "/usr/lib/python3.9/asyncio/base_events.py", line 642, in run_until_complete
    return future.result()
  File "example.py", line 61, in main
    interface.changed()
AttributeError: 'ExampleInterface' object has no attribute 'changed'

This fixes the responsible typo.