Make the simulator programming page self-identify

The url in the tree needs to exactly match the url of the page in
order for the detection of the current page to work. I suspect this
has been broken since I introduced the separate nested 'time' page.

Author: PeterJCLaw

_data/sidebar_tree.yaml

@@ -6,7 +6,7 @@ tree:
   - url: /simulator
     title: Simulator
     tree:
-      - url: /simulator/programming
+      - url: /simulator/programming/
         title: Programming
         tree:
           - url: /simulator/programming/time