Correction to mediation analysis notebook for package name

[asarjou]

Notebook title: mediation_analysis.ipynb Notebook url: https://github.com/pymc-devs/pymc-examples/blob/main/examples/case_studies/mediation_analysis.ipynb

Issue description

The notebook specifies PYMC3 model when it should specify PYMC model in the markdown cell.

Note that this issue tracker is about the contents in the notebooks, if the notebook is instead triggering a bug or error in pymc, please report to https://github.com/pymc-devs/pymc/issues instead

Expected output

Should say: "Define the PyMC3 model and conduct inference"

Proposed solution

Adjustment to Markdown in the notebook