doc.go: add more complex usage example, fix them to omit seconds

fixes #166

doc.go

@@ -7,7 +7,8 @@ Callers may register Funcs to be invoked on a given schedule.  Cron will run
 them in their own goroutines.
 
 	c := cron.New()
-	c.AddFunc("0 30 * * * *", func() { fmt.Println("Every hour on the half hour") })
+	c.AddFunc("30 * * * *", func() { fmt.Println("Every hour on the half hour") })
+	c.AddFunc("30 3-6,20-23 * * *", func() { fmt.Println(".. in the range 3-6am, 8-11pm") })
 	c.AddFunc("CRON_TZ=Asia/Tokyo 30 04 * * * *", func() { fmt.Println("Runs at 04:30 Tokyo time every day") })
 	c.AddFunc("@hourly",      func() { fmt.Println("Every hour") })
 	c.AddFunc("@every 1h30m", func() { fmt.Println("Every hour thirty") })