a small error in docstring of transforms.Normalize (#341)

docstring fix for Normalize: the std sequence should be (S1, ..., Sn).

torchvision/transforms/transforms.py

@@ -94,7 +94,7 @@ class ToPILImage(object):
 
 class Normalize(object):
     """Normalize an tensor image with mean and standard deviation.
-    Given mean: ``(M1,...,Mn)`` and std: ``(M1,..,Mn)`` for ``n`` channels, this transform
+    Given mean: ``(M1,...,Mn)`` and std: ``(S1,..,Sn)`` for ``n`` channels, this transform
     will normalize each channel of the input ``torch.*Tensor`` i.e.
     ``input[channel] = (input[channel] - mean[channel]) / std[channel]``