Remove silenced warnings in for-loops exercise

src/exercises/day-1/for-loops.md

@@ -49,15 +49,14 @@ Hard-code both functions to operate on 3 × 3 matrices.
 Copy the code below to <https://play.rust-lang.org/> and implement the
 functions:
 
-```rust,should_panic
-// TODO: remove this when you're done with your implementation.
-#![allow(unused_variables, dead_code)]
-
+```rust, should_panic
 {{#include for-loops.rs:transpose}}
+    println!("Use matrix {:?}", matrix);
     unimplemented!()
 }
 
 {{#include for-loops.rs:pretty_print}}
+    println!("Use matrix {:?}", matrix);
     unimplemented!()
 }