Remove unnecessary lint allow attrs on example

Author: MikailBag

library/alloc/src/boxed.rs

@@ -99,13 +99,11 @@
 //! pub struct Foo;
 //!
 //! #[no_mangle]
-//! #[allow(improper_ctypes_definitions)]
 //! pub extern "C" fn foo_new() -> Box<Foo> {
 //!     Box::new(Foo)
 //! }
 //!
 //! #[no_mangle]
-//! #[allow(improper_ctypes_definitions)]
 //! pub extern "C" fn foo_delete(_: Option<Box<Foo>>) {}
 //! ```
 //!