Fix typo in autograd docs

Author: apaszke

torch/autograd/__init__.py

@@ -49,7 +49,7 @@ def backward(variables, grad_variables=None, retain_graph=None, create_graph=Non
  The graph is differentiated using the chain rule. If any of ``variables``
  are non-scalar (i.e. their data has more than one element) and require
  gradient, the function additionaly requires specifying ``grad_variables``.
- It should be a sequence of matching length, that containins gradient of
+ It should be a sequence of matching length, that contains gradient of
  the differentiated function w.r.t. corresponding variables (``None`` is an
  acceptable value for all variables that don't need gradient tensors).