Ch. 21.3: remove error ferris from good code

src/ch21-03-graceful-shutdown-and-cleanup.md

@@ -74,7 +74,7 @@ So we need to update the `ThreadPool` `drop` implementation like this:
 
 <Listing file-name="src/lib.rs">
 
-```rust,ignore,does_not_compile
+```rust,ignore
 {{#rustdoc_include ../listings/ch21-web-server/no-listing-04-update-drop-definition/src/lib.rs:here}}
 ```