docs: update docs

Author: code-farmer-i

docs/zh/examples/preview-demo.md

@@ -33,7 +33,12 @@ import VMdPreview from '@kangc/v-md-editor/lib/preview';
 import '@kangc/v-md-editor/lib/style/preview.css';
 import githubTheme from '@kangc/v-md-editor/lib/theme/github.js';
 
-VMdPreview.use(githubTheme);
+// highlightjs
+import hljs from 'highlight.js';
+
+VMdPreview.use(githubTheme, {
+  Hljs: hljs,
+});
 
 const app = creatApp(/*...*/);