message: [backup] fix sample s3 config
diff: @@ -7,4 +7,4 @@ create_backup = 'echo "$BACKUP_NAME"'
  create_for_write = 'echo "s3://$BUCKET/$BACKUP_HANDLE/$FILE_NAME" && exec >&- && aws s3 cp - "s3://$BUCKET/$BACKUP_HANDLE/$FILE_NAME"'
  open_for_read = 'aws s3 cp "$FILE_HANDLE" -'
  save_metadata_line= 'aws s3 cp - "s3://$BUCKET/metadata/$FILE_NAME"'
  -list_metadata_files = 'aws s3 ls s3://$BUCKET/metadata/ | sed -ne "s/.* /p"'
  +list_metadata_files = 'aws s3 ls s3://libra-aldenhu/libra-backup/metadata/ | sed -ne "s/.* //p" | xargs -I{} echo s3://$BUCKET/metadata/{}'
  