add result when the key has expired and it been removed.

I think show the result when the key has expired , what the redis'll return.

In fact, when I run the example, I will get -2 instead of -1, if i've used set command, but the TUTORIAL just talk about the result is -1.

thanks.

Signed-off-by: 晓白 <suyang.sy@alibaba-inc.com>