Fix PEP8 errors

theano/tensor/signal/pool.py

@@ -47,7 +47,8 @@ def max_pool_2d_same_size(input, patch_size):
 
 def pool_2d(input, ds, ignore_border=None, st=None, padding=(0, 0),
             mode='max'):
-    """
+    """Downscale the input by a specified factor
+
     Takes as input a N-D tensor, where N >= 2. It downscales the input image by
     the specified factor, by keeping only the maximum value of non-overlapping
     patches of size (ds[0],ds[1])