Update links to point to tutorial page

[muffinista]

The old link is broken and this seems like the proper one. Addresses #527

[davepagurek]

Hi! The content in /reference/en gets generated automatically from the doc comments in the p5.js repo. To make the changes stick, I think we'll need to make the changes there. At the bottom of reference pages, there's a link to the spot in the p5.js repo where the docs for that page come from.

[muffinista]

Sorry about that! I've redone this work over here: processing/p5.js#7318