Print `hour<-`() result (#531)

datetimes.Rmd

@@ -270,6 +270,7 @@ datetime
 month(datetime) <- 01
 datetime
 hour(datetime) <- hour(datetime) + 1
+datetime
 ```
 
 Alternatively, rather than modifying in place, you can create a new date-time with `update()`. This also allows you to set multiple values at once.