fix(variables5): confine the answer further

let mut number = 3; can lead to a correct answer, so the comment helps to direct the users to the intended answer.

exercises/variables/variables5.rs

@@ -4,7 +4,7 @@
 // I AM NOT DONE
 
 fn main() {
-    let number = "T-H-R-E-E";
+    let number = "T-H-R-E-E"; // don't change this line
     println!("Spell a Number : {}", number);
     number = 3;
     println!("Number plus two is : {}", number + 2);