Fix script blocking (#843)

command/etc/presentation.html

@@ -234,7 +234,7 @@ Use the Command pattern when you want to:
 * Source code http://java-design-patterns.com/patterns/command/
 
     </textarea>
-    <script src="https://gnab.github.io/remark/downloads/remark-latest.min.js">
+    <script src="https://remarkjs.com/downloads/remark-latest.min.js">
     </script>
     <script>
       var slideshow = remark.create();