a doc typo in IAAugmentor (#1308)

tensorpack/dataflow/imgaug/external.py

@@ -40,7 +40,7 @@ class IAAugmentor(ImageAugmentor):
 
     .. code-block:: python
 
-        from imgaug import augmentors as iaa  # this is the aleju/imgaug library
+        from imgaug import augmenters as iaa  # this is the aleju/imgaug library
         from tensorpack import imgaug  # this is not the aleju/imgaug library
         # or from dataflow import imgaug  # if you're using the standalone version of dataflow
         myaug = imgaug.IAAugmentor(