docs: Use perform_enqueued_jobs in an example code

[toshimaru]

I think perform_jobs should be perform_enqueued_jobs,
so I fixed the documentation.

  test "account is charged" do
    perform_enqueued_jobs do
      BillingJob.perform_later(account, product)
    end
    assert account.reload.charged_for?(product)
  end

ref. Testing Rails Applications — Ruby on Rails Guides

[pirj]

Thanks!