Add snippet for RSpec 3 one-liner syntax for subject

[hiasinho]

I added this snippet for the one-liner syntax for setting an expectation on the subject. (see RSpec docs)

I chose itie as the trigger.

RSpec.describe Array do
  describe "with 3 items" do
    subject { [1,2,3] }
    it { is_expected.not_to be_empty }
  end
end

[princemaple]

@hiasinho thank you