Replace broken link in the Lesson 9 page with this one

[mccallion]

In the Lesson 9: Stable Diffusion page, this link is broken:

Summaries + Things Jeremy Says to do + Qs

Here's what I believe is the correct link: https://forums.fast.ai/t/podcast-writeup-summaries-things-jeremy-says-to-do-qs/66194