added 'v4' to error comment

This is to specify in the error message that an IPv4 address is required...for now
removed seqnum definition in crafted packet

The sequence number is not important since this tool will only be targetting fresh connections (sent in reply to a sent SYN packet). The client will take whatever sequence number I tell it, so I'll leave it to the system to generate this. The sequence number will be important if the Reset Attack is aimed at an established connection.
corrected ack num to be seq num + 1

The correct ack number is the remote host's seqnum + 1 to acknowledge the fact that the packet was received, and to notify the remote host this host is waiting on the next (+1) packet