mem.4: Correct the HISTORY section

The history section (added in CSRG) claimed both first appeared in v6.
Looking at the manuals in the TUHS archive, /dev/mem was in v1
and /dev/kmem was introduced in v5.

Reviewed by: imp
Obtained from: OpenBSD
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D44486