docs: fix wrong demonstration of add new markdown-it plugin (#999)

packages/docs/docs/plugin/option-api.md

@@ -215,7 +215,7 @@ module.exports = {
     // Add extra markdown-it plugin
     config
       .plugin('sup')
-        .add(require('markdown-it-sup'))
+        .use(require('markdown-it-sup'))
 
     // Remove internal plugin
     config.plugins.delete('snippet')