examples rss: add mdx file ending

[tordans]

This follows the same pattern used in https://github.com/withastro/astro/blob/main/examples/blog/src/pages/blog.astro#L8. Without adding mdx here, only some blog posts will be part of the rss.xml file.

Changes

This makes sure that the rss.xml lists the same posts as the /blog/ index.

Don't forget a changeset! pnpm exec changeset

Don't know what that means. This change is made via the Github Edit UI.

Testing

I tested the issue in the page I am building ATM.