Fix doctest

Author: newpavlov

ascon-aead/src/lib.rs

@@ -15,6 +15,7 @@
 //! Simple usage (allocating, no associated data):
 //!
 //! ```
+//! # #![cfg(feature = "alloc")]
 //! use ascon_aead::{AsconAead128, Key, Nonce};
 //! use ascon_aead::aead::{Aead, KeyInit};
 //!