example: update CDN asset links (#435)

diff --git a/example/index.html b/example/index.html
index 989b557..97515c6 100644
--- a/example/index.html
+++ b/example/index.html
@@ -4,10 +4,10 @@
     <link rel="stylesheet" href="style.css">
     <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Roboto+Mono|Roboto">
     <link rel="stylesheet" href="https://fonts.googleapis.com/icon?family=Material+Icons+Extended">
-    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.0.1/styles/default.min.css">
+    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.5.1/styles/default.min.css" integrity="sha512-hasIneQUHlh06VNBe7f6ZcHmeRTLIaQWFd43YriJ0UND19bvYRauxthDg8E4eVNPm9bRUhr5JGeqH7FRFXQu5g==" crossorigin="anonymous" referrerpolicy="no-referrer" />
     <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/github-markdown-css/5.1.0/github-markdown-light.min.css" integrity="sha512-zb2pp+R+czM7GAemdSUQt6jFmr3qCo6ikvBgVU6F5GvwEDR0C2sefFiPEJ9QUpmAKdD5EqDUdNRtbOYnbF/eyQ==" crossorigin="anonymous" referrerpolicy="no-referrer" />
-    <script defer src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.0.1/highlight.min.js"></script>
-    <script defer src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.0.1/languages/dart.min.js"></script>
+    <script defer src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.5.1/highlight.min.js" integrity="sha512-yUUc0qWm2rhM7X0EFe82LNnv2moqArj5nro/w1bi05A09hRVeIZbN6jlMoyu0+4I/Bu4Ck/85JQIU82T82M28w==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>
+    <script defer src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.5.1/languages/dart.min.js" integrity="sha512-14QR6tzX5xTNeMJKXzSK+xCquDvtNEr1jM5NlKy/149BBY50Kv70qqxHtzo6zClbtc1gIG7G0CGWXuMgPIMt0g==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>
     <script defer src="app.dart.js"></script>
     <title>Dart Markdown Live Editor</title>
   </head>