notebook: model_selection; update tag

[reshamas]

Notebook title: Model Selection
Notebook url: https://github.com/pymc-devs/pymc-examples/blob/main/examples/generalized_linear_models/GLM-model-selection.ipynb

Issue description

tag is currently "generalized linear models"
need to change it to "generalized linear model" so it matches the category tag of the rest of the GLM notebooks.

Reference

#394