docs: Fix ostree.version -> version
docs: Fix ostree.version -> version

I think originally I had envisioned this as `ostree.version`, but at the last
minute we changed it to just `version`. That's what all of the code uses, so
let's fix the docs.

Closes: #638
Approved by: paulvt

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree