Connecting an always on LED to a raspberry pi
As the question does not specify type of LED is to be used, here are some guidelines that work in general:

Forward voltage of LED, Vf differs by color. Look up the typical voltage for your LED from this Wikipedia link. Green is listed as 1.9 to 4.0 Volts, but they most commonly come in two types: Around 2.1-2.2 Volts, and around 3.2 to 3.6 Volts, depending on junction chemistry.

LED types: 


High intensity "power" or "Illumination" LEDs, most typically white or warm-white, can often be rated for 100 to 500 mA, even more, and they look pretty massive, often flat and square with 5 to 10 mm sides, sometimes 4 pins or contacts instead of 2, sometimes with a thermal pad at the bottom.
"Normal" indicator type through-hole LEDs are usually 3 or 5 mm and rounded, typically rated for 20 mA
Some very small (0402 and smaller) SMD LEDs are rated for just 5 mA.
Infrared LEDs are often rated for much higher (2x to 10x) the current of the above variants.


Nominal current of typical indicator LEDs is generally 20 mA, but they illuminate almost equally well (to the eye) at less than half that value, so let's stay with 5 mA for now. 

So, for green, let is stay with the stated 2.2 Volts. If it were a 3.4 Volt LED it would not light at all, or very dimly, from the 3.3 Volt supply, so you'd know. 

Of the source Voltage, this Vf will be dropped across the LED at nominal current. Hence, the limiting resistor must drop the difference. 

Hence:

R = (V - Vf) / I
  = (3.3 - 2.2) / 0.005
  = 220 Ohms  

This is a standard E12 series resistor value, so we're good.

Using the values mentioned in the question, we get:

R' = (3.3 - 2.2) / 0.02
   = 55 Ohms  

Nearest standard E12 series resistor value = 56 Ohms, so use that.

You are not misunderstanding anything except a possible calculation on the basis of less than 5 mA drive current, and a possible Vf calculation for standard red LEDs.