examples rss: add mdx file ending (#4710)

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.

examples/blog/src/pages/rss.xml.js

@@ -6,5 +6,5 @@ export const get = () =>
 		title: SITE_TITLE,
 		description: SITE_DESCRIPTION,
 		site: import.meta.env.SITE,
-		items: import.meta.glob('./blog/**/*.md'),
+		items: import.meta.glob('./blog/**/*.{md,mdx}'),
 	});