resistor-color: explicitly number the tasks (#1587)

Otherwise it's not clear what "task number one" is.

Author: A5308Y

exercises/concept/resistor-color/.docs/instructions.md

@@ -26,10 +26,10 @@ These colors are encoded as follows:
 - Grey: 8
 - White: 9
 
-The goal of this exercise is to create a way:
-- to look up the numerical value associated with a particular color band
-- to convert the numerical value into a string representing color
-- to list the different band colors
+The goal of this exercise is to create a way to:
+1. look up the numerical value associated with a particular color band
+2. convert the numerical value into a string representing color
+3. list the different band colors
 
 For task number one, you will need the external crate [`int-enum`](https://docs.rs/int-enum/0.5.0/int_enum/) and for task number three, you'll need [`enum-iterator`](https://docs.rs/enum-iterator/1.2.0/enum_iterator/).
 They're both already included in this exercise's `Cargo.toml`.