manual/repo.md: reword bits about the summary file
manual/repo.md: reword bits about the summary file

Closes: #459
Approved by: cgwalters

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