Fix array_read code example error. (#671)

Signed-off-by: Nzhaoyuchen <zhaoyuchen01@baidu.com>

doc/fluid/api_cn/layers_cn.rst

@@ -79,9 +79,9 @@ array_read
 
 .. code-block:: python
 
-    tmp = fluid.layers.zeros(shape=[10],dtype='int32')
+    array = fluid.layers.create_array(dtype='float32')
     i = fluid.layers.fill_constant(shape=[1],dtype='int64',value=10)
-    arr = layers.array_read(tmp,i=i)
+    item = fluid.layers.array_read(array, i)