Fixing numbering in mnist_tutorial.md

[v1vekkumar]

What does this PR do?

Fixes # (issue)

Checklist

• This PR fixes a minor issue (e.g.: typo or small bug) or improves the docs (you can dismiss the other
  checks if that's the case).
• This change is discussed in a Github issue/
  discussion (please add a
  link).
• The documentation and docstrings adhere to the
  documentation guidelines.
• This change includes necessary high-coverage tests.
  (No quality testing = no merge!)

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[8bitmp3]

Thanks @v1vekkumar

Can you check the CLA please https://github.com/google/flax/pull/4311/checks?check_run_id=31748084557

[cgarciae]

Thanks @v1vekkumar !

[cgarciae]

You have to run pre-summit to apply the changes to the notebook:

pip install pre-summit
pre-summit run --all-files
git commit --amend
git push -f

[8bitmp3]

Thanks again @v1vekkumar Please check the CLA https://github.com/google/flax/pull/4311/checks?check_run_id=31748084557