Fix math block of GRUCell in docs (pytorch#572)

Added a blank space between the beginning of the `.. math::` block, otherwise it is displayed as a code block.

Author: jfsantos

torch/nn/modules/rnn.py

@@ -428,6 +428,7 @@ def forward(self, input, hx):
 
 class GRUCell(RNNCellBase):
     r"""A gated recurrent unit (GRU) cell
+    
     .. math::
 
         \begin{array}{ll}