Fix docstring example in HACKING

I61ec91ba4a17c6797f1c626c9b09e4fdb381c357 moves scenario
docstring under the relevant test method, but the reference
to that docstring kept unchanged in HACKING.rst. Besides,
this is no longer a good example of "class level docstring",
so we just pick another one for the example.

Change-Id: I313da5627d283ec4771907d83c485db103fa2702
1 file changed