fix(prejoin) Fix missing lib error

static/prejoin.html

     <!--#include virtual="/base.html" -->
     <link rel="stylesheet" href="css/all.css">
     <script>
+      window.EXCALIDRAW_ASSET_PATH = 'libs/';
       document.addEventListener('DOMContentLoaded', () => {
           if (!JitsiMeetJS.app) {
             return;