update __init__.py

I use pre-commit check all file and report is the passed.
But it is not passed in Travis, so I update __init__.py

python/paddle/v2/dataset/__init__.py

@@ -24,4 +24,4 @@ import sentiment
 __all__ = [
     'mnist', 'imikolov', 'imdb', 'cifar', 'movielens', 'conll05', 'sentiment'
     'uci_housing'
-]
+]