Fix broken link in plugins.mdx

Reviewed-by: Christian Murphy <christian.murphy.42@gmail.com>
Reviewed-by: Titus Wormer <tituswormer@gmail.com>

Closes GH-1314.

docs/advanced/plugins.mdx

@@ -157,4 +157,4 @@ mdx.sync(mdxText, {
 [remarkjs/remark#224]: https://github.com/remarkjs/remark/issues/224
 [visit]: https://github.com/syntax-tree/unist-util-visit
 [retext-smartypants]: https://github.com/retextjs/retext-smartypants
-[create-plugin]: https://unifiedjs.com/create-a-plugin.html
+[create-plugin]: https://unifiedjs.com/learn/guide/create-a-plugin