Update proposals/0304-structured-concurrency.md
Update proposals/0304-structured-concurrency.md

Co-authored-by: Konrad `ktoso` Malawski <7e61f24ab947023f01e6a95efd055bd422a218fc@project13.pl>
apple/swift-evolution,natecook1000/swift-evolution,abertelrud/swift-evolution,apple/swift-evolution,abertelrud/swift-evolution,natecook1000/swift-evolution