Bump @mdx-js/loader from 1.6.16 to 1.6.18 in /site

Bumps [@mdx-js/loader](https://github.com/mdx-js/mdx) from 1.6.16 to 1.6.18.
- [Release notes](https://github.com/mdx-js/mdx/releases)
- [Changelog](https://github.com/mdx-js/mdx/blob/master/changelog.md)
- [Commits](mdx-js/mdx@v1.6.16...v1.6.18)

Signed-off-by: dependabot[bot] <support@github.com>

site/package.json

@@ -9,7 +9,7 @@
   },
   "dependencies": {
     "@mapbox/rehype-prism": "^0.5.0",
-    "@mdx-js/loader": "1.6.16",
+    "@mdx-js/loader": "1.6.18",
     "@mdx-js/mdx": "1.6.16",
     "@next/mdx": "^9.4.4",
     "@tailwindcss/typography": "^0.2.0",