Minor change of docstring example of WeightedRandomSampler (#30846)
Summary:
Previous example
```python
>>> list(WeightedRandomSampler([0.1, 0.9, 0.4, 0.7, 3.0, 0.6], 5, replacement=True))
[0, 0, 0, 1, 0]
```
may seem misleading according to provided weights.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30846
Differential Revision: D19697367
Pulled By: ezyang
fbshipit-source-id: 3d6e3cd0cecb5272a368707ba35bc7acdbd82c30
