F# Hello World: Better asynchronous code

* In the `async` block, await `WriteAsync` instead of ignoring its result. This ensures that the `async` block doesn't return before the write operation is finished.

* Use `Async.StartAsTask` instead of a `Task.Factory`. This is more idiomatic, more concise, and follows the general guideline to avoid `RunSynchronously` when possible.
Merge pull request #1 from Tarmil/patch-1

F# Hello World: Better asynchronous code