networking: document "nc" in addition to "netcat" in netconsole.txt

It always annoyed me that the netconsole documentation didn't give me the
correct command for my distro.  Update it with a command line that actually
works on my Fedora install.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>