ostree init: Fix mode list help string
ostree init: Fix mode list help string

It's archive-z2, not archive-z.

https://bugzilla.gnome.org/show_bug.cgi?id=705849

resin-io/ostree,GNOME/ostree,GNOME/ostree,leandrosansilva/ostree,sujitfulse/ostree,sujitfulse/ostree,gatispaeglis/ostree,resin-io/ostree,GNOME/ostree,leandrosansilva/ostree,gatispaeglis/ostree,resin-io/ostree,sujitfulse/ostree,resin-io/ostree,resin-io/ostree,GNOME/ostree,leandrosansilva/ostree,GNOME/ostree,gatispaeglis/ostree,leandrosansilva/ostree,sujitfulse/ostree,gatispaeglis/ostree,leandrosansilva/ostree,sujitfulse/ostree,gatispaeglis/ostree,gatispaeglis/ostree