torch.library.Library.impl: add missing param in docstring example (#98619)
previously this was missing the callable
```
>>> my_lib = Library("aten", "IMPL")
>>> def div_cpu(self, other):
>>> return self * (1 / other)
>>> my_lib.impl("div.Tensor", "CPU")
# ^ missing `div_cpu` here
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98619
Approved by: https://github.com/ezyang
