docs: fix bogus override for GIConv
docs: fix bogus override for GIConv

GIConv is a typedef'ed pointer, don't claim it is a struct.
https://bugzilla.gnome.org/show_bug.cgi?id=779501

johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,johne53/MB3Glib