when reporting logging messages for prune targets, correctly report the target URL

[deitch]

This was causing weird log messages, like:

DEBU[0000] pruning 0 files from target &{{s3  %!s(*url.Userinfo=<nil>) *********-backup-mumbai /mysql/dumps-frequent  %!s(bool=false) %!s(bool=false)   } %!s(bool=false)    }  run=7f4831e8-7247-4674-8bd2-4ccaefa848a