Sync the resistor-color-duo exercise's docs with the latest data. (exercism#358)

Author: ErikSchierboom

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

@@ -17,16 +17,16 @@ 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, and