fix: confine the user further in variable exercises

We want to teach a specific lesson. To ensure that we do, let's try to
provide more clarity on what the user should not do.

exercises/variables/variables5.rs

@@ -4,7 +4,7 @@
 // I AM NOT DONE
 
 fn main() {
-    let number = "3";
+    let number = "3"; // don't change this line
     println!("Number {}", number);
     number = 3;
     println!("Number {}", number);