Use todo!() instead of ???

exercises/17_tests/tests3.rs

@@ -29,8 +29,8 @@ mod tests {
         // TODO: This test should check if the rectangle has the size that we
         // pass to its constructor.
         let rect = Rectangle::new(10, 20);
-        assert_eq!(???, 10); // Check width
-        assert_eq!(???, 20); // Check height
+        assert_eq!(todo!(), 10); // Check width
+        assert_eq!(todo!(), 20); // Check height
     }
 
     // TODO: This test should check if the program panics when we try to create