Update 'Clojure in Small Pieces' to a working link (#5363)

* Update 'Clojure in Small Pieces' to a working link

The original is now a 404.

* Add unglued source link for 'Clojure in Small Pieces'
Co-authored-by: NDavid Ordás <davorpatech@gmail.com>
Co-authored-by: NDavid Ordás <davorpatech@gmail.com>

books/free-programming-books.md

@@ -1253,7 +1253,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Clojure Cookbook](https://github.com/clojure-cookbook/clojure-cookbook)
 * [Clojure Distilled Beginner Guide](http://yogthos.github.io/ClojureDistilled.html)
 * [Clojure for the Brave and True](http://www.braveclojure.com)
-* [Clojure in Small Pieces](http://daly.axiom-developer.org/clojure.pdf) - Rich Hickey (PDF)
+* [Clojure in Small Pieces](https://web.archive.org/web/20201013022918/http://daly.axiom-developer.org/clojure.pdf) - Rich Hickey, Timothy Daly (PDF) [(:card_file_box: *unglued*)](https://unglue.it/work/489419/)
 * [Clojure Koans](http://clojurekoans.com)
 * [Clojure Programming](https://en.wikibooks.org/wiki/Clojure_Programming) - Wikibooks
 * [ClojureScript Koans](http://clojurescriptkoans.com)