Fix function_exported_3 doctest
//! or directly registering the `code` function like
//!
//! ```
-//! # use liblumen_alloc::erts::term::Atom;
+//! # use liblumen_alloc::erts::term::prelude::Atom;
//! let module = Atom::try_from_str("erlang").unwrap();
//! let function = Atom::try_from_str("self").unwrap();
//! let arity = 0;
