[Docs] Minor example error in workers

[rromanchuk]

I don't think you meant to use the symbol :user_id, instead you probably meant the local scoped var user_id

class NotificationJob
  include Sidekiq::Worker

  def perform(user_id)
    Rollbar.scope!(:person => { :id => :user_id })

    # If this next line causes an exception, the reported exception (which will
    # be reported by Rollbar's standard Sidekiq instrumentation) will also
    # include the above person information.
    Notification.send_to_user(user_id)
  end
end

It's funny, i noticed i had the user "user_id" causing a majority of the error in my workers, turns out i'm an idiot and just copy and pasted from this example.

[rromanchuk]

#596