Update demo site URL

Author: mvz

index.html

@@ -63,9 +63,9 @@ <h2>
 
         <h2><a name="demo-site" class="anchor" href="#demo-site"></a>Demo site</h2>
 
-        <p>You can <a href="https://demo-publify.herokuapp.com/">give Publify a try</a></p>
+        <p>You can <a href="https://publify-demo.fly.dev/">give Publify a try</a></p>
 
-        <p>The login / password <a href="https://demo-publify.herokuapp.com/admin">to the admin</a>
+        <p>The login / password <a href="https://publify-demo.fly.dev/admin">to the admin</a>
         are:</p>
 
         <ul>