update error information (#30277)

python/paddle/vision/datasets/folder.py

@@ -134,7 +134,7 @@ class DatasetFolder(Dataset):
                                is_valid_file)
         if len(samples) == 0:
             raise (RuntimeError(
-                "Found 0 files in subfolders of: " + self.root + "\n"
+                "Found 0 directories in subfolders of: " + self.root + "\n"
                 "Supported extensions are: " + ",".join(extensions)))
 
         self.loader = default_loader if loader is None else loader