Fix grammatical errors on the PyTorch Contribution Guide (#61818)
Summary:
## What does the PR do?
- Fix grammatical errors on the PyTorch Contribution Guide page.
## Changes [Screenshots]
> Note:
> 1. The changes are highlighted in each screenshot.
> 2. Could not load CSS while testing locally, hope that is not an issue as all the changes are made on the content.
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Pull Request resolved: https://github.com/pytorch/pytorch/pull/61818
Reviewed By: dzhulgakov
Differential Revision: D29775606
Pulled By: mrshenli
fbshipit-source-id: 3f3bfdeede341f784b72dfe55da9ba8bdce1192a
