summary: Fix a typo
summary: Fix a typo

The --gpg-sign parameter will be used for signing the summary, not the
commit. This probably was copy-pasted from the ostree commit command.

Closes: #320
Approved by: cgwalters

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