Fix log loss doc (#1704)

doc/fluid/api_cn/layers_cn/log_loss_cn.rst

@@ -30,8 +30,8 @@ log_loss
 ..  code-block:: python
 
     import paddle.fluid as fluid
-    label = fluid.layers.data(name='label', shape=[1], dtype='int64')
-    prob = fluid.layers.data(name='prob', shape=[10], dtype='float32')
+    label = fluid.data(name='label', shape=[None, 1], dtype='float32')
+    prob = fluid.data(name='prob', shape=[None, 1], dtype='float32')
     cost = fluid.layers.log_loss(input=prob, label=label)