move ref_slice example and add historical note (#365)

src/patterns/structural/small-crates.md

@@ -33,15 +33,15 @@ We should take advantage of this tooling, and use smaller, more fine-grained dep
 
 ## Examples
 
-The [`ref_slice`](https://crates.io/crates/ref_slice) crate provides functions
-for converting `&T` to `&[T]`.
-
 The [`url`](https://crates.io/crates/url) crate provides tools for working with
 URLs.
 
 The [`num_cpus`](https://crates.io/crates/num_cpus) crate provides a function to
 query the number of CPUs on a machine.
 
+The [`ref_slice`](https://crates.io/crates/ref_slice) crate provides functions
+for converting `&T` to `&[T]`. (Historical example)
+
 ## See also
 
 - [crates.io: The Rust community crate host](https://crates.io/)