fix readme testing section

[edmartins]

In the Testing section of the README.md has a test example as bellow:

it 'prevents duplicate jobs from being scheduled' do
  SidekiqUniqueJobs.use_config(enabled: true) do
    expect(described_class.perform_async(1)).not_to eq(nil)
    expect(described_class.perform_async(1)).to eq(nil)
  end
end

The problem is if the first perform_async runs quickly before the second, the test will not pass.
In another application I had problems performing the tests this way, being that only sometimes the test passed.
Changing the first expect with perform_in the result is as expected.