Fix latex error in heaviside docs (#44481)
Summary:
This fixes a `katex` error I was getting trying to build the docs:
```
ParseError: KaTeX parse error: Undefined control sequence: \0 at position 55: …gin{cases}
```
This failure was introduced in https://github.com/pytorch/pytorch/issues/42523.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44481
Reviewed By: colesbury
Differential Revision: D23627700
Pulled By: mruberry
fbshipit-source-id: 9cc09c687a7d9349da79a0ac87d6c962c9cfbe2d
