Update flippercloud.io docs for ActiveSupportCacheStore adapter: `expires_in` kwarg is deprecated

[tomczak-mateusz]

Hey @jnunemaker, not sure if it's a valid place to open this issue, but could not find any repo related to Flipper docs: https://www.flippercloud.io/docs.

We have integrated flipper-active_support_cache_store adapter according to the documentation:
https://www.flippercloud.io/docs/adapters/active-support-cache-store

Flipper.configure do |config|
  config.use Flipper::Adapters::ActiveSupportCacheStore, Rails.cache, expires_in: 5.minutes
end

After integration, we could observe multiple deprecation warnings: DEPRECATION WARNING: The expires_in kwarg is deprecated for Flipper::Adapters::ActiveSupportCacheStore and will be removed in the next major version. Please pass in expires in as third argument instead.

I think the same issue might be related to doc as well: https://www.flippercloud.io/docs/optimization#activesupportcachestore but did not test this way of integration:

Not using kwarg fixes this warning:

Flipper.configure do |config|
  config.use Flipper::Adapters::ActiveSupportCacheStore, Rails.cache, 5.minutes
end

I suggest to update docs to match non deprecated integration.