libvaladoc: change Valadoc link in HTML doclet

At present the HTML output has a footer stating: 'Generated by Valadoc'. The word Valadoc links to valadoc.org. It makes sense to change this link to https://wiki.gnome.org/Projects/Valadoc because that is the tool that generated the documentation. valadoc.org is a site holding pre-generated documentation for many libraries Vala can use, but is not the home page of the tool that generates the documentation.

The advantage of this change is people following the link will have a better understanding of the Vala tools available.