Update gnome-doc.el

Isn't the version from gtk-doc more up to date?
http://git.gnome.org/browse/gtk-doc/plain/tools/gtk-doc.el
Merge remote-tracking branch 'elmarco/patch-1'
Rename gnome-doc.el to gtk-doc.el for consistency with gtk-doc

This file comes from the gtk-doc module, in gtk-doc/tools.  So, call it the same here.
Remove gnome-doc from the README, as it is integrated now