Problem downloading the dataset for F#'s Image Classification Model Training sample

When running this sample, I get the exception below, indicating there's some problem when downloading the `flower_photos_small_set.zip` dataset:

```
Unhandled exception. System.Net.WebException: No such host is known. No such host is known.
---> System.Net.Http.HttpRequestException: No such host is known.
---> System.Net.Sockets.SocketException (11001): No such host is known.
at System.Net.Http.ConnectHelper.ConnectAsync(String host, Int32 port, CancellationToken cancellationToken)
--- End of inner exception stack trace ---
at System.Net.Http.ConnectHelper.ConnectAsync(String host, Int32 port, CancellationToken cancellationToken)
at System.Net.Http.HttpConnectionPool.ConnectAsync(HttpRequestMessage request, Boolean allowHttp2, CancellationToken cancellationToken)
at System.Net.Http.HttpConnectionPool.CreateHttp11ConnectionAsync(HttpRequestMessage request, CancellationToken cancellationToken)
at System.Net.Http.HttpConnectionPool.GetHttpConnectionAsync(HttpRequestMessage request, CancellationToken cancellationToken)
at System.Net.Http.HttpConnectionPool.SendWithRetryAsync(HttpRequestMessage request, Boolean doRequestAuth, CancellationToken cancellationToken)
at System.Net.Http.RedirectHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
at System.Net.Http.HttpClient.FinishSendAsyncUnbuffered(Task`1 sendTask, HttpRequestMessage request, CancellationTokenSource cts, Boolean disposeCts)
at System.Net.HttpWebRequest.SendRequest()
at System.Net.HttpWebRequest.GetResponse()
--- End of inner exception stack trace ---
at System.Net.HttpWebRequest.GetResponse()
at System.Net.WebClient.GetWebResponse(WebRequest request)
at System.Net.WebClient.DownloadBits(WebRequest request, Stream writeStream)
at System.Net.WebClient.DownloadFile(Uri address, String fileName)
at Program.downloadZippedImageSetAsync@29.Invoke(Unit unitVar) in C:\Users\anvelazq\Desktop\mlnetsamples\samples\fsharp\getting-started\DeepLearning_ImageClassification_Training\ImageClassification.Train\Program.fs:line 34
at Microsoft.FSharp.Control.AsyncPrimitives.CallThenInvoke[T,TResult](AsyncActivation`1 ctxt, TResult result1, FSharpFunc`2 part2) in E:\A\_work\130\s\src\fsharp\FSharp.Core\async.fs:line 398
at Microsoft.FSharp.Control.Trampoline.Execute(FSharpFunc`2 firstAction) in E:\A\_work\130\s\src\fsharp\FSharp.Core\async.fs:line 109
--- End of stack trace from previous location where exception was thrown ---
at Microsoft.FSharp.Control.AsyncResult`1.Commit() in E:\A\_work\130\s\src\fsharp\FSharp.Core\async.fs:line 350
at Microsoft.FSharp.Control.AsyncPrimitives.RunSynchronouslyInCurrentThread[a](CancellationToken cancellationToken, FSharpAsync`1 computation) in E:\A\_work\130\s\src\fsharp\FSharp.Core\async.fs:line 882
at Microsoft.FSharp.Control.AsyncPrimitives.RunSynchronously[T](CancellationToken cancellationToken, FSharpAsync`1 computation, FSharpOption`1 timeout) in E:\A\_work\130\s\src\fsharp\FSharp.Core\async.fs:line 890
at Microsoft.FSharp.Control.FSharpAsync.RunSynchronously[T](FSharpAsync`1 computation, FSharpOption`1 timeout, FSharpOption`1 cancellationToken) in E:\A\_work\130\s\src\fsharp\FSharp.Core\async.fs:line 1154
at Program.main(String[] argv) in C:\Users\anvelazq\Desktop\mlnetsamples\samples\fsharp\getting-started\DeepLearning_ImageClassification_Training\ImageClassification.Train\Program.fs:line 73
```

Instead, if I run the [C# version of this same tutorial](https://github.com/dotnet/machinelearning-samples/tree/master/samples/csharp/getting-started/DeepLearning_ImageClassification_Training/ImageClassification.Train), the dataset is correctly downloaded, and so if I take that dataset and put it inside where the F# sample is trying to find it, then the F# tutorial works correctly.

PS: I checked that the URL where it's trying to download the dataset is the same in both versions of the tutorial, and it is. So I'm not sure where the problem is.

Inizia da samples/fsharp/getting-started/DeepLearning_ImageClassification_Training/ImageClassification.Train/Program.fs, in particolare da downloadZippedImageSetAsync alla riga 34, e confrontalo con il corrispondente punto di ingresso per il download dell'esempio C#. Esegui l'esempio F# e verifica che flower_photos_small_set.zip venga scaricato correttamente e che il workflow di training venga completato senza l'errore di risoluzione dell'host.

Scritto dal modello di indicizzazione a partire dal testo della issue.