samples/pktgen: Correct comments about the thread config

They all claimed to be two CPU examples using eth1, eth2 but
that is only true in one case!

Rob Jones pointed out spelling and grammar errors here, which I've
also corrected.

Cc: Rob Jones <rob.jones@codethink.co.uk>
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>