fix: 修改md文件，优化html使用示例

README.md

@@ -41,8 +41,7 @@ fetch('https://your-api.com/track', {
 <script src="dist/main.js"></script>
 <script>
   // 你的库会挂载在 window.UtmTracker
-  var UtmTracker = window.UtmTracker;
-  var utmParams = UtmTracker.get();
+  var utmParams = window.UtmTracker.get();
   console.log('UTM参数:', utmParams);
 </script>
 ```