BlinkyHeadless documentation is unnecessarily using async modifier

[nshenoy]

In the *Initialize the GPIO pin" section of the BlinkyHeadless.md documentation, it declares the method as private async void InitGPIO(). I think the original author may have intended to use await GpioController.GetDefaultAsync() in the sample instead of calling the synchronous GpioController.GetDefault(). That would complicate the sample for beginners though. I think the docs should match the sample code in the repo and simply remove the async modifier.