Fixed incorrect example of long-polling in README

Example of long-polling in Streaming Responses section was not working as expected.

The example first called `Sinatra::Base#body` with `Sinatra::Helpers::Stream` then again with ["subscribed"]`.
This resulted in instant rendering of `"subscribed"` as body of the response and closing of the
connection.

I fixed the incorrect example, see pull request.

Regards,
detomastah