Use expire_after instead of expires_in for session_store in README

[tnantoka]

Thank you for providing a great Gem!

expires_in is used for session_store in README#Usage. It does not seem to work.
Is'nt it mistaken with expire_after?

For Rails default CookieStore, we use expire_after.

Both CookieStore and RedisStore inherits Rack::Session::Abstract::Persisted.
It has expire_after option but not expires_in.

Thanks.

[tubbo]

Hey @tnantoka, thanks for your contribution! Definitely a mistake on my part when I updated the docs for Sentinel configuration, when you look at the source for Redis::Store we do accept multiple options including :expires_in, :expire_in, and :expires_after, but then looking at the source for Rack::Session::Abstract::ID that you posted, it seems that the rack session store will only accept :expires_after. I just wanted to make them all more consistent. :)

Let me know if you find any other documentation issues!