fix typo (#412)

torchvision/transforms/transforms.py

@@ -107,7 +107,7 @@ class ToPILImage(object):
 
 
 class Normalize(object):
-    """Normalize an tensor image with mean and standard deviation.
+    """Normalize a tensor image with mean and standard deviation.
     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]``