fix the gather cn

doc/fluid/api_cn/layers_cn/gather_cn.rst

@@ -45,7 +45,7 @@ gather
   
   import paddle.fluid as fluid
   x = fluid.layers.data(name='x', shape=[-1, 5], dtype='float32')
-  index = fluid.layers.data(name='index', shape=[-1, 1], dtype='int32')
+  index = fluid.layers.data(name='index', shape=[1], dtype='int32')
   output = fluid.layers.gather(x, index)