doc: remove out of date links and info from packet mmap

The packet_mmap documentation had links to no longer existing web
sites; replace with other site which has similar example.

Support for packet mmap has been in mainline versions of libpcap
for several years.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>