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/variables3.rs

@@ -6,6 +6,6 @@
 fn main() {
     let x = 3;
     println!("Number {}", x);
-    x = 5;
+    x = 5; // don't change this line
     println!("Number {}", x);
 }