man: netsniff-ng: refine ethtool section

Just add yet another example for ethtool, nothing more.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
netsniff-ng: remove set_sockopt_hwtimestamp call

For TX this call is completly useless and has no effect whatsoever.
Therefore, remove it. For the RX part, this call makes perfect
sense, not for the other one currently.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>