Model Specification Fix in Discrete Choice Notebook

[NathanielF]

Discrete Choice Notebook:
https://www.pymc.io/projects/examples/en/latest/generalized_linear_models/GLM-discrete-choice_models.html:

Issue description

Following discussion on PyMC discourse i want to fix the model specification to improve parameter identification
https://discourse.pymc.io/t/inconsistencies-w-r-in-discrete-choice-and-random-utility-pymc-example/16488/4

Expected output

If applicable, describe what should happen instead.

Proposed solution

Updated model specification