pktgen: Correct documentation of module name and command

Drop the '.o' suffix so this text properly covers both the
built-in and modular cases.

'insmod pktgen' obviously won't work; the command should be modprobe.

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