Update additional-pages.md

Add missing `use` in example

guides/recipes/additional-pages.md

@@ -21,6 +21,8 @@ Admin pages must use the `Beacon.LiveAdmin.PageBuilder` module, which provides t
 
 ```elixir
 defmodule MyAppWeb.Admin.DocumentsLive.Index do
+  use Beacon.LiveAdmin.PageBuilder
+
   @impl true
   def menu_link(_prefix, _live_action), do: {:root, "Documents"}
 
@@ -43,4 +45,4 @@ defmodule MyAppWeb.Admin.DocumentsLive.Index do
 end
 ```
 
-You'll notice the only different from a regular LiveView is the `menu_link/2` callback, which is used to define the menu item for the page.
+You'll notice the only different from a regular LiveView is the `menu_link/2` callback, which is used to define the menu item for the page.