Fix story Resistor Color Duo

See #1471

The current stories for Resistor Color and Resistor Color Duo raised some questions, as collected in the issue mentioned above.
Recap of the wishlist:
- Avoid the word 'encoding' 
- Don't make the listing of the color and its value look like a dictionary.
- Make it more explicit that a color band has a numeric value _and_ a position.
 
This commit is addressing these.

exercises/resistor-color-duo/description.md

@@ -2,19 +2,9 @@ If you want to build something using a Raspberry Pi, you'll probably use _resist
 
 * Each resistor has a resistance value.
 * Resistors are small - so small in fact that if you printed the resistance value on them, it would be hard to read.
-To get around this problem, manufacturers print color-coded bands onto the resistors to denote their resistance values. Each band acts as a digit of a number. For example, if they printed a brown band (value 1) followed by a green band (value 5), it would translate to the number 15.
+To get around this problem, manufacturers print color-coded bands onto the resistors to denote their resistance values. Each band has a position and a numeric value. For example, if they printed a brown band (value 1) followed by a green band (value 5), it would translate to the number 15.
 
 In this exercise, you are going to create a helpful program so that you don't have to remember the values of the bands. The program will take two colors as input, and output the correct number.
 
-The band colors are encoded as follows:
-
-- Black: 0
-- Brown: 1
-- Red: 2
-- Orange: 3
-- Yellow: 4
-- Green: 5
-- Blue: 6
-- Violet: 7
-- Grey: 8
-- White: 9
+The colors are mapped to the numbers from 0 to 9 in the sequence:
+Black - Brown - Red - Orange - Yellow - Green - Blue - Violet - Grey - White