Fix reader creator in mnist

demo/mnist/api_train_v2.py

@@ -33,7 +33,7 @@ def main():
     trainer.train(
         reader=paddle.reader.batched(
             paddle.reader.shuffle(
-                paddle.dataset.mnist.train_creator(), buf_size=8192),
+                paddle.dataset.mnist.train(), buf_size=8192),
             batch_size=32),
         cost=cost,
         parameters=parameters,

python/paddle/v2/dataset/mnist.py

@@ -45,7 +45,7 @@ def reader_creator(image_filename, label_filename, buffer_size):
         m.terminate()
         l.terminate()
 
-    return reader()
+    return reader
 
 
 def train():