Update broken link (#5309)

* remove broken link

* update broken link
Co-authored-by: NSam Hsiao <samhsiao@pilottv.com.tw>

books/free-programming-books.md

@@ -2689,7 +2689,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Rust for Rubyists](https://web.archive.org/web/20190520171322/http://www.rustforrubyists.com/book/index.html) - Steve Klabnik
 * [The Rust Programming Language](http://doc.rust-lang.org/book/)
 * [The Rustonomicon](https://doc.rust-lang.org/nomicon/)
-* [Why Rust?](http://www.oreilly.com/programming/free/files/why-rust.pdf) (PDF)
+* [Why Rust?](https://www.oreilly.com/content/why-rust/)
 
 
 ### Sage