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.
1.
![Change1](https://user-images.githubusercontent.com/20442648/126077764-39fd8b78-524f-407d-bc39-c93167bd10a7.PNG)
2.
![Change2](https://user-images.githubusercontent.com/20442648/126077766-9dd7dc61-ef06-41d0-a7e5-cfd179ece0cd.PNG)
3.
![Change3](https://user-images.githubusercontent.com/20442648/126077767-2c2e05e4-09fc-403a-a18e-9b108651a5f8.PNG)
4.
![Change4](https://user-images.githubusercontent.com/20442648/126077769-ad755db6-3afa-457b-b95c-9f6c6281f828.PNG)
5.
![Change5](https://user-images.githubusercontent.com/20442648/126077770-a7759dee-7f90-4b9e-a07c-4dec4ca934d0.PNG)
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![Change6](https://user-images.githubusercontent.com/20442648/126077772-0474e58d-c0c8-4156-b56f-808d225c38e7.PNG)
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![Change7](https://user-images.githubusercontent.com/20442648/126077774-d48382a7-5379-49a4-a8d2-b478fabf0bf0.PNG)
8.
![Change8](https://user-images.githubusercontent.com/20442648/126077777-fd743825-8dd7-4cb9-a22c-233e5fa085a6.PNG)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61818
Reviewed By: dzhulgakov
Differential Revision: D29775606
Pulled By: mrshenli
fbshipit-source-id: 3f3bfdeede341f784b72dfe55da9ba8bdce1192a