fix wrong link

README.md

@@ -8,7 +8,7 @@
     <a href="https://knowstreaming.com">产品官网</a> | 
     <a href="https://github.com/didi/KnowStreaming/releases">下载地址</a> | 
     <a href="https://doc.knowstreaming.com">文档资源</a> | 
-    <a href="https://ddemo.knowstreaming.com">体验环境</a> 
+    <a href="https://demo.knowstreaming.com">体验环境</a> 
 </p>
 
 <p align="center">