The example was incorrectly using parantheses around the list of columns, so
just drop them.
Reported-By: Robert Haas
Discussion: https://postgr.es/m/CA%2BTgmoZZEMAqWMAfvLHZnK57SoxOutgvE-ALO94WsRA7zZ7wyQ%40mail.gmail.com