Add中week增加类型转换 (#554)

cannot use week (type float64) as type time.Duration in argument to t.Add

eBook/04.8.md

@@ -40,7 +40,7 @@ func main() {
 	fmt.Println(time.Now()) // Wed Dec 21 09:52:14 +0100 RST 2011
 	// calculating times:
 	week = 60 * 60 * 24 * 7 * 1e9 // must be in nanosec
-	week_from_now := t.Add(week)
+	week_from_now := t.Add(time.Duration(week))
 	fmt.Println(week_from_now) // Wed Dec 28 08:52:14 +0000 UTC 2011
 	// formatting times:
 	fmt.Println(t.Format(time.RFC822)) // 21 Dec 11 0852 UTC