fix(prejoin) fix css loading path

static/prejoin.html

@@ -4,7 +4,7 @@
     <meta http-equiv="content-type" content="text/html;charset=utf-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <!--#include virtual="/base.html" -->
-    <link rel="stylesheet" href="../css/all.css">
+    <link rel="stylesheet" href="css/all.css">
     <script>
       document.addEventListener('DOMContentLoaded', () => {
           if (!JitsiMeetJS.app) {