docs/interop/firmware.json: convert "Example" section

docs/interop/firmware.json: convert "Example" section

Since commit 3c5f6114d9ff ("qapi: remove "Example" doc section")
the "Example" section is not valid anymore.
It has been replaced by the "qmp-example" directive.

This was not detected earlier as firmware.json was not validated.
As this validation is about to be added, adapt firmware.json.

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Message-ID: <20240719-qapi-firmware-json-v6-3-c2e3de390b58@linutronix.de>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>