Clarify confusing comment in strings2.rs

Fixes #2230
Replaced vague wording "this function" with the explicit function name "is_a_color_word" to improve clarity for beginners.

exercises/09_strings/strings2.rs

@@ -1,4 +1,4 @@
-// TODO: Fix the compiler error in the `main` function without changing this function.
+// TODO: Fix the compiler error in the `main` function without changing the `is_a_color_word` function.
 fn is_a_color_word(attempt: &str) -> bool {
     attempt == "green" || attempt == "blue" || attempt == "red"
 }