Fix image_classification syntax

fluid/image_classification/train.py

@@ -33,7 +33,7 @@ add_arg('lr',               float, 0.1,                  "set learning rate.")
 add_arg('lr_strategy',      str,   "piecewise_decay",    "Set the learning rate decay strategy.")
 add_arg('model',            str,   "SE_ResNeXt50_32x4d", "Set the network to use.")
 add_arg('enable_ce',        bool,  False,                "If set True, enable continuous evaluation job.")
-add_arg('data_dir'          str,   "./data/ILSVRC2012",  "The ImageNet dataset root dir.")
+add_arg('data_dir',         str,   "./data/ILSVRC2012",  "The ImageNet dataset root dir.")
 # yapf: enable
 
 model_list = [m for m in dir(models) if "__" not in m]