docs: fixes Setup guide (#25)

Just a small change in the example moving from /setup (blank site) to /guide/setup (actual docs for installation)

Author: DominikAngerer

docs/content/en/guide/usage.md

@@ -34,7 +34,7 @@ Inside `hello.mdx`, add some markdown content:
   This a Nuxt MDX tomato.
 </section>
 
-<nuxt-link to="/setup">
+<nuxt-link to="/guide/setup">
   back to setup page &rarr;
 </nuxt-link>
 ```