Use app_domain in Redirector example

In the current redirector example, "scribe.rip" is hardcoded as the
destination. This patch simply changes that to use the app_domain
environment variable, so people wanting to use a community instance
aren't mistakenly redirected to the main scribe.rip instance.

src/pages/home/index_page.cr

@@ -57,7 +57,9 @@ class Home::IndexPage < MainLayout
           end
           li do
             strong "Redirect to: "
-            code "https://scribe.rip/$1"
+            code "https://"
+            code app_domain
+            code "/$1"
           end
           li do
             strong "Pattern type: "