docs: Rename a parameter in a GObject how-to example
docs: Rename a parameter in a GObject how-to example

Make it obvious the parameter is not related to AClass.

https://bugzilla.gnome.org/show_bug.cgi?id=744060

cention-sany/glib,ieei/glib,tchakabam/glib,mzabaluev/glib,endlessm/glib,cention-sany/glib,mzabaluev/glib,tchakabam/glib,johne53/MB3Glib,MathieuDuponchelle/glib,johne53/MB3Glib,johne53/MB3Glib,gale320/glib,johne53/MB3Glib,gale320/glib,endlessm/glib,tchakabam/glib,MathieuDuponchelle/glib,cention-sany/glib,tchakabam/glib,mzabaluev/glib,ieei/glib,endlessm/glib,Distrotech/glib,cention-sany/glib,MathieuDuponchelle/glib,ieei/glib,mzabaluev/glib,Distrotech/glib,MathieuDuponchelle/glib,ieei/glib,tchakabam/glib,johne53/MB3Glib,johne53/MB3Glib,gale320/glib,MathieuDuponchelle/glib,gale320/glib,Distrotech/glib,gale320/glib,cention-sany/glib,Distrotech/glib,mzabaluev/glib,ieei/glib,endlessm/glib,Distrotech/glib,endlessm/glib