Update AppVeyor URL.

readme.md

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 # Avalonia
 [![Gitter](https://badges.gitter.im/Join Chat.svg)](https://gitter.im/AvaloniaUI/Avalonia?utm_campaign=pr-badge&utm_content=badge&utm_medium=badge&utm_source=badge)
 
-[![Build status](https://ci.appveyor.com/api/projects/status/hubk3k0w9idyibfg/branch/master?svg=true)](https://ci.appveyor.com/project/Avalonia/Avalonia/branch/master)
+[![Build status](https://ci.appveyor.com/api/projects/status/hubk3k0w9idyibfg/branch/master?svg=true)](https://ci.appveyor.com/project/AvaloniaUI/Avalonia/branch/master)
 
 A multi-platform .NET UI framework. It can run on Windows, Linux, Mac OS X, iOS and Android.