[stzn/the-swift-programming-language-jp] Issue opened: #598 ConcurrencyのExample修正

以下に変更

extension TemperatureLogger { func convertFahrenheitToCelsius() { for i in measurements.indices { measurements[i] = (measurements[i] - 32) * 5 / 9 } } }

https://github.com/swiftlang/swift-book/pull/344