qapi: fix example of dump-guest-memory

Example output lacks mandatory member @paging.  Provide it.

Signed-off-by: Victor Toso <victortoso@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-Id: <20220328140604.41484-15-victortoso@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>