alloc: Workaround hidden doctest line

A small workaround for rust-lang#136899,
rustdoc's invalid_rust_codeblocks was not handling this well in 2024.
This may be needed when migrating to 2024 when building with stage0.

Author: ehuss

library/alloc/src/boxed.rs

@@ -24,7 +24,7 @@
 //! Creating a recursive data structure:
 //!
 //! ```
-//! ##[allow(dead_code)]
+//! # #[allow(dead_code)]
 //! #[derive(Debug)]
 //! enum List<T> {
 //!     Cons(T, Box<List<T>>),