fix sample code error of fluid.dygraph.NaturalExpDecay (#2219)

fix sample code error of fluid.dygraph.NaturalExpDecay

doc/fluid/api_cn/dygraph_cn/NaturalExpDecay_cn.rst

@@ -53,12 +53,14 @@ NaturalExpDecay
     import paddle.fluid as fluid
     base_lr = 0.1
     with fluid.dygraph.guard():
+        emb = fluid.dygraph.Embedding([10, 10])
         sgd_optimizer = fluid.optimizer.SGD(
                 learning_rate=fluid.dygraph.NaturalExpDecay(
-                      learning_rate=base_lr,
-                      decay_steps=10000,
-                      decay_rate=0.5,
-                      staircase=True))
+                    learning_rate=base_lr,
+                    decay_steps=10000,
+                    decay_rate=0.5,
+                    staircase=True),
+                parameter_list=emb.parameters())