The stop is in the motor itself. If the motor is working correctly, you remove the wiper arms and let the motor come to its stop. Then reinstall the arms accordingly. If the motor has stop problems, then you will have to go into the motor or replace it with a good used or new (expensive - about $250) motor.
If you only want to adjust the resting point of one wiper arm, you can remove the nut that holds it on the pivot shaft, pull the arm socket completely off the pin, turn it slightly one way or the other, and then push it back onto the shaft and replace the nut. Based on the fine gauge of the "teeth" on the shaft (located just below the nut retaining threads at the tip) you should be able to adjust the clocking of the wiper arm in about 15 degree increments.