swaplabel: wrong version number in check

$ swaplabel /dev/sda2
swaplabel: /dev/sda2: unsupported swap version '1'

The mkswap does not allow any other version, so swaplabel(8) must be
wrong.

Reference: https://github.com/karelzak/util-linux/blob/master/disk-utils/mkswap.c#L520
Signed-off-by: Sami Kerola <kerolasa@iki.fi>