fix stan model

[OriolAbril]

Description

Checklist

• [x] Does the PR follow official PR format?
• [x] Is the code style correct (follows pylint and black guidelines)?

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📚 Documentation preview 📚: https://arviz--2297.org.readthedocs.build/en/2297/

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (22c0dcb) 86.70% compared to head (e15e79f) 86.70%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2297 +/- ## ======================================= Coverage 86.70% 86.70% ======================================= Files 122 122 Lines 12640 12640 ======================================= Hits 10959 10959 Misses 1681 1681 ```

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