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.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/tutorials/3672

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 1 New Failure

As of commit e61e858 with merge base 7f8b6dc ():

NEW FAILURE - The following job has failed:

• link check on PR / linkChecker (gh)
  Process completed with exit code 1.

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