s3-mirror: fix missing trailing /
    
    This is causing f31 updates to not be synced. The cron job reports:
    
    Subject: Cron <s3-mirror@mm-backend01> /usr/local/bin/lock-wrapper s3sync-updates-current "/usr/local/bin/s3-sync-path.sh /pub/fedora/linux/updates/31/Everything/x86_64/os" 2>&1 |
            /usr/local/bin/nag-once s3-updates-current.sh 1d 2>&1
    
    Syntax: /usr/local/bin/s3-sync-path.sh /pub/path/to/sync/
    NOTE! Path must end with a trailing /
    
    Signed-off-by: Kevin Fenzi <kevin@scrye.com>