Update README.md

Author: iceopen

demo/debugcharts/README.md

@@ -11,7 +11,7 @@ import _ "github.com/mkevac/debugcharts"
 ```
 
 ## 使用案例：
-```
+```golang
 package main
 
 import (
@@ -60,4 +60,4 @@ func main() {
 	log.Printf("you can now open http://localhost:8080/debug/charts/ in your browser")
 	select {}
 }
-```
+```