Use consistent capitalization, and fix a missed line (we duplicate the
qemu-img synopses in too many places).

Fixes: 1899bf47 (qemu-img: Add -F shorthand to convert)
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210921142812.2631605-1-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>