Fix metadata source_url for simple-linked-list exercise (#1649)

Previous web.archive.org link pointed to the book homepage.
Change it to point to simple-linked-list page.

Author: tejasbubane

exercises/simple-linked-list/metadata.yml

@@ -1,4 +1,4 @@
 ---
 blurb: "Write a simple linked list implementation that uses Elements and a List"
 source: "Inspired by 'Data Structures and Algorithms with Object-Oriented Design Patterns in Ruby', singly linked-lists."
-source_url: "https://web.archive.org/web/20160414064110/http://www.brpreiss.com/books/opus8/"
+source_url: "https://web.archive.org/web/20160731005714/http://brpreiss.com/books/opus8/html/page96.html"