This example doesn't compile but wasn't marked as such

Author: carols10cents

Diff for: src/ch15-01-box.md

@@ -128,7 +128,7 @@ Listing 15-3:
 
 <span class="filename">Filename: src/main.rs</span>
 
-```rust,ignore
+```rust,ignore,does_not_compile
 use crate::List::{Cons, Nil};
 
 fn main() {