Fix TOC links in PageVisibility Tutorial

[kohenkatz]

The PageVisibility tutorial has multiple article sections, all of which have the id set to toc-topic. As you might imagine, this makes the navigation not actually work - all of the TOC links lead to the first section (except for the "Introduction" and "References" links).

There are also multiple occurrences of id="toc-topic-subtopic", which, while not shown in the table of contents at the moment, could still be used for hand-made links to subtopics if they were fixed as well.