Fix expression for the variance in ZINB docstring

[aloctavodia]

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📚 Documentation preview 📚: https://pymc--7200.org.readthedocs.build/en/7200/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.29%. Comparing base (a06081e) to head (3a4b471).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7200      +/-   ##
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+ Coverage   91.87%   92.29%   +0.42%     
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  Files         100      100              
  Lines       16874    16874              
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+ Hits        15503    15574      +71     
+ Misses       1371     1300      -71     

Files	Coverage Δ
pymc/distributions/mixture.py	95.08% <ø> (ø)

... and 3 files with indirect coverage changes

[ricardoV94]

@aloctavodia do you have a reference / simple derivation to be able to review non-blindly :D?

[aloctavodia]

I don't have a reference. I got this expression a while ago while working on PreliZ, I found it difficult to get a reference at that time. Then I forgot about it, and found it again this weekend while working on PreliZ. I could try to re-derived... What I did was test it empirically, by generating random samples and computing the variance for a few combinations of parameters.

[ricardoV94]

No worries then!