This one got me right away as well.... Remember that the peak-to-peak voltage is twice the peak voltage. So you have to divide his given voltage by 2 then convert to RMS.
From wikipedia:
Peak values can be calculated from RMS values from the above formula, which implies Vp = VRMS × √2, assuming the source is a pure sine wave.
Thus the peak value of the mains voltage in the USA is about 120 × √2, or about 170 volts. The peak-to-peak voltage, being twice this, is about 340 volts.
Thanks,
Jason