Ohm's Law states that voltage is equal to the current multiplied by the resistance, or V =
IR . Subtracting 2.2V from 3.3V of our supply voltage, and dividing the resulting value by
.020 Amps gives 55 Ohms. 55 Ohms isn't a standard value, but 56 is, so you can use this.
But, you can always use a higher resistance value, and the LED will just be less bright.
The resistor used in this project had a value of 100 Ohms.