Fix links in the footer

There are three links in the footer that point to the current page right
now. They are:

  1. Class Hierarchy Diagram
  2. Errors
  3. Markers

Fix these links so that they point to the correct pages instead.

  1. Class Hierarchy Diagram should point to graphs/class.html
  2. Errors should point to reports/errors.html
  3. Markers should point to reports/markers.html

Fixes phpDocumentor/template.clean#5
<https://github.com/phpDocumentor/template.clean/issues/5>