change docstring

Author: shuoweil

bigframes/dataframe.py

@@ -1929,7 +1929,7 @@ def to_pandas_batches(
                 over the default size limit of 10 GB.
 
         Returns:
-            Iterable[pandas.DataFrame]:
+            blocks.PandasBatches:
                 An iterable of smaller dataframes which combine to
                 form the original dataframe. Results stream from bigquery,
                 see https://cloud.google.com/python/docs/reference/bigquery/latest/google.cloud.bigquery.table.RowIterator#google_cloud_bigquery_table_RowIterator_to_arrow_iterable