go vet fix

sync.go

@@ -233,7 +233,7 @@ func (db *Database) Sync(remote SyncClient) error {
 				if book, err := db.GetBook(bookid); err == nil {
 					if files, err := db.GetBookFiles(bookid); err == nil {
 						if err := remote.SendBook(book, files); err != nil {
-							log.Printf("SendBook(%d): %v", id, err)
+							log.Printf("SendBook(%s): %v", id, err)
 						}
 					}
 				}