TCP reliable data transfer duplicated "snd.nxt" and grammar fix

[kaskales]

There was a duplicated "snd.nxt" variable with the incorrect conjugation of "is" which should be "are" since it's plural. I imagine one of the "snd.nxt" variables should be changed to "snd.wnd" since this segment is being sent and should be noted as such in the sending window as well.