fix gru doc (#6296)

Author: cocoshe

docs/api/paddle/nn/GRU_cn.rst

@@ -3,7 +3,7 @@
 GRU
 -------------------------------
 
-.. py:class:: paddle.nn.GRU(input_size, hidden_size, num_layers=1, direction="forward", dropout=0., time_major=False, weight_ih_attr=None, weight_hh_attr=None, bias_ih_attr=None, bias_hh_attr=None, name=None)
+.. py:class:: paddle.nn.GRU(input_size, hidden_size, num_layers=1, direction="forward", time_major=False, dropout=0.0, weight_ih_attr=None, weight_hh_attr=None, bias_ih_attr=None, bias_hh_attr=None, name=None)