website: Fix broken link in preview API docs
This fixes a broken link that was in the header of each page of the preview API documentation, pointing to https://gnome.pages.gitlab.gnome.org/tracker/docs/ which gives a 404.
This fixes a broken link that was in the header of each page of the preview API documentation, pointing to https://gnome.pages.gitlab.gnome.org/tracker/docs/ which gives a 404.