Minor path bug in plot_model_card.py

[tuscland]

Hello,

When I generate the model card using the python script provided in this example, the permutation importances figure is referenced using an absolute path, instead of a path relative to the repository:

model_card.add_permutation_importances(
    importances,
    X_test.columns,
    plot_file=Path(local_repo) / "importance.png",
    plot_name="Permutation Importance",
)

This results in the following output (in README.md):

![Permutation Importance](/var/folders/pc/r_g178qx7tx56zn8zlgqft9h0000gn/T/skops-m2dcspf7/importance.png)

Instead, the plot_file argument should just be "importance.png".