fix doc

Author: yuyu2172

chainercv/datasets/imagenet/imagenet_det_bbox_dataset.py

@@ -32,8 +32,8 @@ class ImagenetDetBboxDataset(GetterDataset):
 
     Args:
         data_dir (string): Path to the root of the training data. If this is
-            :obj:`auto`, this class will automatically download data for you
-            under :obj:`$CHAINER_DATASET_ROOT/pfnet/chainercv/imagenet`.
+            :obj:`auto`, this uses
+            :obj:`$CHAINER_DATASET_ROOT/pfnet/chainercv/imagenet` by default.
         split ({'train', 'val'}): Select a split of the
             dataset.
         return_img_label (bool): If :obj:`True`, this dataset returns