block/archipelago: Fix typo in qemu_archipelago_truncate()

Fix a typo introduced by 94c80a438c85f2c19698547fbb115ea46d80c5f1

Signed-off-by: Chrysostomos Nanakos <cnanakos@grnet.gr>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>