Fix typos in PolyaGamma's docstring

[aleicazatti]

Fix typos and change a detail in PolyaGamma's docstring.

• A backslash and a backtick were missing in the math code block for the mean and the deployment looked like this:
  

• The example plot wasn't deployed and it was the only distribution with plt.style.use('seaborn-darkgrid'). This commit imports arviz and uses plt.style.use('arviz-darkgrid') instead.

...

Checklist

• Explain important implementation details 👆
• Make sure that the pre-commit linting/style checks pass.

---

📚 Documentation preview 📚: https://pymc--6672.org.readthedocs.build/en/6672/

[codecov]

Codecov Report

Merging #6672 (4d18474) into main (fa82fef) will increase coverage by 2.49%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6672      +/-   ##
==========================================
+ Coverage   89.50%   91.99%   +2.49%     
==========================================
  Files          94       95       +1     
  Lines       15942    16180     +238     
==========================================
+ Hits        14269    14885     +616     
+ Misses       1673     1295     -378     

Impacted Files	Coverage Δ
pymc/distributions/continuous.py	97.74% <ø> (+0.03%)	⬆️
pymc/logprob/basic.py	99.15% <100.00%> (ø)

... and 18 files with indirect coverage changes

[ricardoV94]

The plot is still not visible in the preview: https://pymcio--6672.org.readthedocs.build/projects/docs/en/6672/api/distributions/generated/pymc.PolyaGamma.html

CC @OriolAbril

[OriolAbril]

The rtd log has ModuleNotFoundError: No module named 'polyagamma'. The code snippet uses the polyagamma library but I guess it is not part of the docs conda env, so it can't actually generate the plot

[ricardoV94]

The rtd log has ModuleNotFoundError: No module named 'polyagamma'. The code snippet uses the polyagamma library but I guess it is not part of the docs conda env, so it can't actually generate the plot

Right, it's an optional dependency. Shall we merge as is, or is it a problem?

[OriolAbril]

I wouldn't merge as is. Either add the dependency to the doc build env, or remove the code to generate the plot (the code isn't shown, so the only value of it being in the docstring is the plot it generates)