dt-bindings: power: Fix example for power-domain

The change that converted the domain-idle-states DT bindings to the
json-schema, forgot to update the corresponding example for the
power-domain binding. Let's fix it.

Fixes: 3261227d ("dt-bindings: power: Convert domain-idle-states bindings to json-schema")
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>