Update dataset_dict.py (#6932)

shape returns (number of rows, number of columns)

src/datasets/dataset_dict.py

@@ -169,7 +169,7 @@ def column_names(self) -> Dict[str, List[str]]:
 
     @property
     def shape(self) -> Dict[str, Tuple[int]]:
-        """Shape of each split of the dataset (number of columns, number of rows).
+        """Shape of each split of the dataset (number of rows, number of columns).
 
         Example: