pktgen: Fix grammar errors and some poor wording in documentation

Thanks to Rob Jones for suggesting some of the changes.

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>
1 file changed