[doc] typo (#66754)
Summary:
This PR fixes a typo in the `torch/autograd/function.py` doc
-----------------------
Additionally, the example at https://pytorch.org/docs/master/autograd.html#torch.autograd.Function doesn't quite compile:
```
'builtin_function_or_method' object has no attribute 'exp'
```
even though `i.exp()` is a valid function if `i` is a tensor.
I changed it to:
```
result = torch.exp(i)
```
but python doesn't like it either:
```
TypeError: exp(): argument 'input' (position 1) must be Tensor, not builtin_function_or_method
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66754
Reviewed By: albanD
Differential Revision: D31729400
Pulled By: soulitzer
fbshipit-source-id: eef783bcdc8d4693a8b7f1ab581e948abc0f9b94
