Correct the usage of fc in the example of dynamic_lstm's doc

Author: Yibing Liu

python/paddle/v2/fluid/layers/nn.py

@@ -321,7 +321,7 @@ def dynamic_lstm(input,
 
             hidden_dim = 512
             forward_proj = fluid.layers.fc(input=input_seq, size=hidden_dim * 4,
-                                           act='tanh', bias_attr=True)
+                                           act=None, bias_attr=None)
             forward, _ = fluid.layers.dynamic_lstm(
                 input=forward_proj, size=hidden_dim * 4, use_peepholes=False)
     """