Update readme.md of next-mdx to allow typescript file extensions for pages (vercel#32830)

Author: cmdcolin

packages/next-mdx/readme.md

@@ -69,7 +69,7 @@ const withMDX = require('@next/mdx')({
   extension: /\.mdx?$/,
 })
 module.exports = withMDX({
-  pageExtensions: ['js', 'jsx', 'md', 'mdx'],
+  pageExtensions: ['js', 'jsx', 'ts', 'tsx', 'md', 'mdx'],
 })
 ```