Update/Standardize nodebook: GLM-negative-binomial-regression

[AmitKus]

1. Fix KeyError
2. Update the notebook based on style guide

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[codecov]

Codecov Report

Merging #4158 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4158   +/-   ##
=======================================
  Coverage   88.75%   88.75%           
=======================================
  Files          89       89           
  Lines       14039    14039           
=======================================
+ Hits        12460    12461    +1     
+ Misses       1579     1578    -1     

Impacted Files	Coverage Δ
pymc3/distributions/continuous.py	92.93% <0.00%> (+0.11%)	⬆️

[AlexAndorra]

All good, thanks @AmitKus !