Notebook GLM-negative-binomial-regression needs update to PyMC v5

[wesleyboelrijk]

Notebook title: GLM-negative-binomial-regression
Notebook url: https://github.com/pymc-devs/pymc-examples/blob/main/examples/generalized_linear_models/GLM-negative-binomial-regression.ipynb

Issue description

The GLM-negative-binomial-regression needs an update to the PyMC v5 version.