errors3: Add a comment to prevent changing the wrong line

Author: mo8it

exercises/13_error_handling/errors3.rs

@@ -19,6 +19,7 @@ fn main() {
     let mut tokens = 100;
     let pretend_user_input = "8";
 
+    // Don't change this line.
     let cost = total_cost(pretend_user_input)?;
 
     if cost > tokens {