'family' option is not part of type 2 table and if user tries to use it
as such QEMU will error out with an unknow option error.
Drop it from docs lest it confuse users.

Fixes: b155eb1d ("smbios: document cmdline options for smbios type 2-4, 17 structures")
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <1558448611-315074-1-git-send-email-imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>