exercises(resistor-color-duo): sync docs (#597)

Reflect the upstream changes [1][2].

[1] exercism/problem-specifications@9c864c448fc1
[2] exercism/problem-specifications@f6e92949b582

exercises/practice/resistor-color-duo/.docs/instructions.md

@@ -17,17 +17,17 @@ The program will take color names as input and output a two digit number, even i
 
 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
+- black: 0
+- brown: 1
+- red: 2
+- orange: 3
+- yellow: 4
+- green: 5
+- blue: 6
+- violet: 7
+- grey: 8
+- white: 9
 
 From the example above:
-brown-green should return 15
+brown-green should return 15, and
 brown-green-violet should return 15 too, ignoring the third color.