Add correct output for loo_kfold()

[jordandeklerk]

This corrects the ELPDData object to display the correct output when using the loo_kfold() function. Also cleans up the docstring a bit.

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Resolves #189

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📚 Documentation preview 📚: https://arviz-stats--190.org.readthedocs.build/en/190/

[codecov-commenter]

Codecov Report

❌ Patch coverage is 20.00000% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.58%. Comparing base (a40917b) to head (2266347).
⚠️ Report is 117 commits behind head on main.

Files with missing lines	Patch %	Lines
src/arviz_stats/utils.py	20.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #190      +/-   ##
==========================================
+ Coverage   62.64%   70.58%   +7.93%     
==========================================
  Files          14       39      +25     
  Lines        1925     4375    +2450     
==========================================
+ Hits         1206     3088    +1882     
- Misses        719     1287     +568     

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