Explicitely point out example's dependency (#1312)

* Explicitely point out example dependency

Save the next visitor to that docs page the trouble of having to figure
out why a Path<Uuid> extractor results in a cryptic error by default.

* Update axum/src/extract/path/mod.rs

Co-authored-by: David Pedersen <david.pdrsn@gmail.com>

Co-authored-by: David Pedersen <david.pdrsn@gmail.com>

axum/src/extract/path/mod.rs

@@ -25,6 +25,10 @@ use std::{
 ///
 /// # Example
 ///
+/// These examples assume the `serde` feature of the [`uuid`] crate is enabled.
+///
+/// [`uuid`]: https://crates.io/crates/uuid
+///
 /// ```rust,no_run
 /// use axum::{
 ///     extract::Path,