Use relative import for Aura CSS

Author: web-padawan

dev/aura.html

@@ -5,7 +5,7 @@
     <meta http-equiv="X-UA-Compatible" content="IE=edge" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>Aura</title>
-    <link rel="stylesheet" href="/packages/aura/aura.css">
+    <link rel="stylesheet" href="../packages/aura/aura.css">
     <script type="module">
       window.Vaadin ||= {};
       window.Vaadin.featureFlags ||= {};