watchtower/multi: fix logging in wtclient

watchtower/wtdb/client_session.go

@@ -162,7 +162,7 @@ func (b *BackupID) Decode(r io.Reader) error {
 }
 
 // String returns a human-readable encoding of a BackupID.
-func (b *BackupID) String() string {
+func (b BackupID) String() string {
 	return fmt.Sprintf("backup(%x, %d)", b.ChanID, b.CommitHeight)
 }