Update accessibility.md (#660)

The link to the explanation of landmarks, was not up to date, suggesting this http://www.scottohara.me/blog/2018/03/03/landmarks.html provides an up to date and detailed explanation.

content/docs/accessibility.md

@@ -133,7 +133,7 @@ Also use landmark elements and roles, such as `<main>` and `<aside>`, to demarca
 
 Read more about the use of these elements to enhance accessibility here:
 
-- [Deque University - HTML 5 and ARIA Landmarks](https://dequeuniversity.com/assets/html/jquery-summit/html5/slides/landmarks.html)
+- [Accessible Landmarks](http://www.scottohara.me/blog/2018/03/03/landmarks.html)
 
 ### Programmatically managing focus