defmt/docs: fix link to mdbook
//! A highly efficient logging framework that targets resource-constrained devices, like
//! microcontrollers.
//!
-//! For more details check the book at https://defmt.ferrous-systems.com
+//! For more details check the book at <https://defmt.ferrous-systems.com>
#![cfg_attr(not(target_arch = "x86_64"), no_std)]
#![warn(missing_docs)]
