403 in async-and-await sample

Hi,

just tried the [sample](https://docs.microsoft.com/en-gb/previous-versions/visualstudio/visual-studio-2012/hh300224(v=vs.110)) which has 2 issues:

1) In the first few steps, it says to choose WPF App and name it... so I choose a wpf app for .NET Core 5.0, follow the guidelines a little further and it says make sure you are targeting .NET Framework 4.5. Why didn't it say that earlier?

2) I pasted in the bottom example in the end, run it with F5 then got a 403 error:

System.Net.Http.HttpRequestException: 'Response status code does not indicate success: 403 (Forbidden).' at

wait SumPageSizesAsync();

Also, a .NET Core 5 example would be nice.