docs: update link to markdown-to-jsx homepage

[bartvanandel]

Reasons for making this change

I noticed when browsing your example page that the link to the markdown-to-jsx package website was dead. This PR updates it to the current url, as displayed in their repo on GitHub. This was updated on their side in quantizor/markdown-to-jsx@342f587, 29 days ago today.

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
• I'm adding or updating code
  • I've added and/or updated tests. I've run npm run test:update to update snapshots, if needed.
  • I've updated docs if needed
  • I've updated the changelog with a description of the PR
• I'm adding a new feature
  • I've updated the playground with an example use of the feature