Fix docstring of the ZeroInflatedPoisson distribution

[cscheffler]

What is this PR about?

The parameterization uses the variable mu but the docstring uses $\mu$ in some cases and $\theta$ in other cases. The fix replaces all $\theta$ with $\mu$ to match the argument name for this distribution.

Checklist

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Docs / Maintenance

• Fixed ZeroInflatedPoisson docstring, replacing theta with mu

[codecov]

Codecov Report

Merging #6213 (50adef3) into main (d47dac0) will decrease coverage by 3.58%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6213      +/-   ##
==========================================
- Coverage   93.58%   90.00%   -3.59%     
==========================================
  Files         101      101              
  Lines       22136    22120      -16     
==========================================
- Hits        20716    19908     -808     
- Misses       1420     2212     +792     

Impacted Files	Coverage Δ
pymc/backends/ndarray.py	79.27% <ø> (-0.19%)	⬇️
pymc/distributions/discrete.py	99.21% <ø> (ø)
pymc/backends/base.py	87.95% <100.00%> (+0.84%)	⬆️
pymc/sampling.py	82.62% <100.00%> (+0.14%)	⬆️
pymc/tests/backends/fixtures.py	78.30% <100.00%> (+0.31%)	⬆️
pymc/tests/distributions/test_timeseries.py	0.00% <0.00%> (-96.96%)	⬇️
pymc/tests/step_methods/hmc/test_quadpotential.py	0.00% <0.00%> (-95.82%)	⬇️
pymc/distributions/timeseries.py	30.38% <0.00%> (-61.07%)	⬇️
pymc/model_graph.py	66.66% <0.00%> (-12.29%)	⬇️
pymc/step_methods/hmc/quadpotential.py	73.76% <0.00%> (-6.94%)	⬇️
... and 2 more