Fix example url in documentation

[yboulkaid]

The link to the full example in the README was pointing to a 404.

This PR corrects the url by linking to config/initializers/sidekiq.rb instead of config/sidekiq.rb.

I also took the liberty to remove the "and hopefully working" from the copy, to make the documentation
feel more assertive. I can revert that change if desired.

[mhenrixon]

Thanks a bunch! I will create a separate example repository that uses all the middleware in combination, haven't gotten around to it yet.

[yboulkaid]

Sounds great! And congrats on releasing v7 btw :)