pktgen: document the latest pktgen usage options

Currently, the pktgen.rst documentation doesn't cover the latest pktgen
sample usage options such as count and IPv6, and so on. Also, this
documentation includes the old sample scripts which are no longer use
because it was removed by the commit a4b6ade8

 ("samples/pktgen :
remove remaining old pktgen sample scripts")

Thus, this commit documents pktgen sample usage using the latest options
and removes old sample scripts, and fixes a minor typo.

Signed-off-by: Juhee Kang <claudiajkang@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>