update project URL to be the GitHub repository

Ix.NET/Source/Directory.build.props

@@ -6,7 +6,7 @@
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <Authors>.NET Foundation and Contributors</Authors>
     <PackageIconUrl>http://go.microsoft.com/fwlink/?LinkId=261274</PackageIconUrl>
-    <PackageProjectUrl>http://go.microsoft.com/fwlink/?LinkId=261273</PackageProjectUrl>
+    <PackageProjectUrl>https://github.com/dotnet/reactive</PackageProjectUrl>
     <PackageLicenseUrl>https://raw.githubusercontent.com/Reactive-Extensions/Rx.NET/master/Ix.NET/Source/license.txt</PackageLicenseUrl>
     <PackageRequireLicenseAcceptance>true</PackageRequireLicenseAcceptance>    
     <SignAssembly>true</SignAssembly>