doc: add go playground demo

docs/datetime_zh-CN.md

@@ -223,10 +223,10 @@ import (
 func main() {
     now := time.Now()
 
-	after1Year := AddYear(now, 1)
+	after1Year := datetime.AddYear(now, 1)
 	diff1 := after1Year.Sub(now)
 
-	before1Year := AddYear(now, -1)
+	before1Year := datetime.AddYear(now, -1)
 	diff2 := before1Year.Sub(now)
 
 	fmt.Println(diff1)
@@ -1103,7 +1103,6 @@ func main() {
 }
 ```
 
-
 ### <span id="IsLeapYear">IsLeapYear</span>
 
 <p>验证是否是闰年。</p>