Fix GitHub URL for MNIST archive download

[patrocinio]

Updated URL from: https://github.com/pytorch/tutorials/raw/main/_static/

To: https://github.com/pytorch/tutorials/raw/refs/heads/main/_static/

Fixes issue #3569

Description

Checklist

• [ ] The issue that is being fixed is referred in the description (see above "Fixes #ISSUE_NUMBER")
• [ ] Only one issue is addressed in this pull request
• [ ] Labels from the issue that this PR is fixing are added to this pull request
• [ ] No unnecessary issues are included into this pull request.