DOC: Outdated link to matplotlib documentation in documentation for heatmap

In the documentation for the cbar_kws argument of heatmap, the link given for matplotlib's colorbar's arguments is https://matplotlib.org/stable/api/figure_api.html#matplotlib.figure.Figure.colorbar which sends to the wrong page.

The right page to see colorbar arguments is now https://matplotlib.org/stable/api/_as_gen/matplotlib.figure.Figure.colorbar.html.