Radon Stan case study linking to ipynb instead of html

[avehtari]

In case-studies.qmd other case studies link to a html, but on line 914 radon case study links to ipynb which doesn't render to something useful in a web browser https://github.com/stan-dev/stan-dev.github.io/blob/e37e400843cfcdc6c002649f63f42df1260f708e/learn-stan/case-studies.qmd#L914

I don't know how to fix this, so making an issue. Maybe @mitzimorris knows what to do?