trafgen: minor fix to help text for option "--no-sock-mem"

Pushed last commit a bit too fast.  The help text described that
the single letter option for "--no-sock-mem" were "-m", which is
wrong the right option is "-A" (only help txt, correct in the code
and manual).

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>