openshift-apps/flatpak-indexer: fix wrong URL to upstream repo
    
    One of the references to the upstream repository referenced
    https://pagure.io/flatpak-indexer, but at least for now, the code
    lives at https://github.com/owtaylor/flatpak-indexer.
    
    Signed-off-by: Owen W. Taylor <otaylor@fishsoup.net>