gvarianttype: Fix typos
Saying that a GtkWidget is a GtkWidget is trivially true, but not the point we were trying to make here.
Fixes: b5c07063 "docs: Use code for class names in links"
Saying that a GtkWidget is a GtkWidget is trivially true, but not the point we were trying to make here.
Fixes: b5c07063 "docs: Use code for class names in links"