rust: alloc: align Debug implementation for Box with Display

Ensure consistency between `Debug` and `Display` for `Box` by
updating `Debug` to match the new `Display` style.

Acked-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Guangbo Cui <2407018371@qq.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/tencent_1FC0BC283DA65DD81A8A14EEF25563934E05@qq.com


[ Reworded title. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>