Use ordered_univariate in documentation example of NormalMixture

[mrpg]

What is this PR about?
The documentation example of a normal mixture used an outdated path in the transform argument of pm.Normal.

Checklist

• Explain important implementation details 👆
• Make sure that the pre-commit linting/style checks pass.
• Link relevant issues (preferably in nice commit messages)
• Are the changes covered by tests and docstrings?
• Fill out the short summary sections 👇

Major / Breaking Changes

• None

New features

• None

Bugfixes

• None

Documentation

• Instead of pm.transforms.ordered, an Ordered is instantiated and used in its place.

Maintenance

• None

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📚 Documentation preview 📚: https://pymc--6842.org.readthedocs.build/en/6842/

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[codecov]

Codecov Report

Merging #6842 (e6392bd) into main (5c600c7) will not change coverage.
Report is 5 commits behind head on main.
The diff coverage is n/a.

❗ Current head e6392bd differs from pull request most recent head 54a0998. Consider uploading reports for the commit 54a0998 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
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  Coverage   92.03%   92.03%           
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  Files          96       96           
  Lines       16378    16378           
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  Hits        15074    15074           
  Misses       1304     1304           

Files Changed	Coverage Δ
pymc/distributions/mixture.py	94.97% <ø> (ø)

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[ricardoV94]

Thanks @mrpg!