Update doc for ecs_service for using non-default cluster

[jedipunkz]

I confirmed that it can be tested with the following description.

describe ecs_service('example'), cluster: 'example' do
  it { should exist }
  it { should be_active }
end

The test results are as follows.

ecs_service 'example'
  is expected to exist
  is expected to be active

Finished in 0.15046 seconds (files took 3.15 seconds to load)
2 examples, 0 failures

So we should update docs relatet to 'ecs_service' test.

[k1LoW]

Hi @jedipunkz ! Thank you for your commit !
Could you update doc or merge master for fixing test fail?