Fix docs links (#481)

axum/src/docs/handlers_intro.md

@@ -1,5 +1,5 @@
 In axum a "handler" is an async function that accepts zero or more
-["extractors"](#extractors) as arguments and returns something that
+["extractors"](crate::extract) as arguments and returns something that
 can be converted [into a response](crate::response).
 
 Handlers is where your application logic lives and axum applications are built

axum/src/handler/mod.rs

@@ -67,6 +67,8 @@
 //! This error doesn't tell you _why_ your function doesn't implement
 //! [`Handler`]. It's possible to improve the error with the [`debug_handler`]
 //! proc-macro from the [axum-debug] crate.
+//!
+//! [axum-debug]: https://docs.rs/axum-debug
 
 use crate::{
     body::{box_body, BoxBody},