Fix typo in arxiv.org links

Changes `https://arxiv.org/abs/1502.03509s` to `https://arxiv.org/abs/1502.03509`.

Author: dvmazur

flows.py

@@ -56,7 +56,7 @@ def forward(self, inputs, cond_inputs=None):
 
 class MADESplit(nn.Module):
     """ An implementation of MADE
-    (https://arxiv.org/abs/1502.03509s).
+    (https://arxiv.org/abs/1502.03509).
     """
 
     def __init__(self,
@@ -130,7 +130,7 @@ def forward(self, inputs, cond_inputs=None, mode='direct'):
 
 class MADE(nn.Module):
     """ An implementation of MADE
-    (https://arxiv.org/abs/1502.03509s).
+    (https://arxiv.org/abs/1502.03509).
     """
 
     def __init__(self,