docs: update HOWTO for 3.x -> 4.x versioning

[ Upstream commit e4144fe5d47c91c92d36cdbd5f31ed8d6e3a57ab ]

The HOWTO document needed updating for the new kernel versioning.

Signed-off-by: Mario Carrillo <mario.alfredo.c.arevalo@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>