docs: Replace awkward `timm` link with the expected one
# What does this PR do?

Replace https://github.com/rwightman/pytorch-image-models/tree/master/timm with https://github.com/rwightman/pytorch-image-models. 

## Reasoning
1. The URL has the hardcoded `master` branch, despite the `timm` being branch being renamed to `main` nowadays.
2. The URL points to the `timm` folder for some reason, when linking to the root, i.e. where a README is visible, is much more sensible.

## Before submitting
- [x] This PR fixes a typo or improves the docs

## Who can review?

Documentation: @sgugger

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I just keep running into small issues here and there! More than happy to help fix them, though.

- Tom Aarsen
_The documentation is not available anymore as the PR was closed or merged._