docs: Fixed 2 links in the docs along with some minor fixes

[Sai-Suraj-27]

What does this PR do?

Here, the link to Python 3.8 might keep changing due to changes in setup.py. So, I changed it to a permalink.

transformers/CONTRIBUTING.md

Line 135 in 4c040ab

You'll need **[Python 3.8](https://github.com/huggingface/transformers/blob/main/setup.py#L426)** or above to contribute to 🤗 Transformers. Follow the steps below to start contributing:

Here, the link to testing_utils.py is not working in docs, as the docs are linked to this file. So, I just changed the link to the file's github link

transformers/CONTRIBUTING.md

Line 349 in 4c040ab

More environment variables and additional information can be found in the [testing_utils.py](src/transformers/testing_utils.py).

Fixes # (issue)

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@amyeroberts

[amyeroberts]

Thanks for updating! Two small comments

[amyeroberts]

Thanks!