docs: Fix broken link on Getting Started with MDX page (#33148)

* Fix broken link

* Remove ending slash

* Update docs/docs/mdx/getting-started.md

Co-authored-by: Ward Peeters <[email protected]>

Author: minimalsm

docs/docs/mdx/getting-started.md

@@ -67,5 +67,5 @@ Alternatively, you may be looking to configure an existing blog site to use MDX.
 
 ## What's next?
 
-Go check out the [writing MDX guide](/docs/how-to/routing/mdx/writing-pages) to find out what else you can do
+Go check out the [writing MDX guide](/docs/mdx/writing-pages/) to find out what else you can do
 with Gatsby and MDX.