fix format specifier in snapshot storing

mu_map/training/lib.py

@@ -78,7 +78,7 @@ class AbstractTraining:
                 self.store_snapshot("val_min")
 
             if epoch % self.snapshot_epoch == 0:
-                self.store_snapshot(f"{epoch:0d{len(str(self.epochs))}}")
+                self.store_snapshot(f"{epoch:0{len(str(self.epochs))}d}")
 
             for param in self.training_params:
                 if param.lr_scheduler is not None: