docs: Update active-links.md (#2207)

* Update active-links.md

Add Documentation regarding RouteLink "Active" state through the use of slot properties

* Update packages/docs/guide/essentials/active-links.md

---------

Co-authored-by: Eduardo San Martin Morote <posva@users.noreply.github.com>

Author: claushellsing

packages/docs/guide/essentials/active-links.md

@@ -75,4 +75,4 @@ const router = createRouter({
 })
 ```
 
-See [Extending RouterLink](../advanced/extending-router-link) for more advanced customization techniques.
+See [Extending RouterLink](../advanced/extending-router-link) for more advanced customization techniques using the `v-slot` API.