Tweak MVC diagram link in Workshop site

[Fixes railsbridge#386]

Author: tjgrathwell

sites/en/workshop/diagrams.mw

@@ -2,7 +2,7 @@
 
 * MVC: http://github.com/alexch/mission/raw/master/mvc.png
 
-* MVC: http://ruby.railstutorial.org/chapters/a-demo-app#sec-mvc_in_action
+* MVC: https://www.railstutorial.org/book/demo_app#sec-mvc_in_action
 
 * REST: