Fix broken rst of torch.nn.utils.spectral_norm and others (#15995)

Fix broken rst of torch.nn.utils.spectral_norm and others (#15995)

Summary:
- Currently, the [rst](https://pytorch.org/docs/stable/nn.html#torch.nn.utils.spectral_norm) looks broken, at least in my browser. So I fixed it.
- I thought a subscript may be needed to the left W in the definition.
- A few typos fixed.

crcrpar
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15995

Differential Revision: D13649888

Pulled By: soumith

fbshipit-source-id: 00a2c3b043c7c8ebdd9fc2bf77ba27ae695fee3f