Fix release notes link to work by default

It also makes it work in the case where `PackageProjectUrl` isn't the github.com repository itself.

Author: AArnott

Directory.Build.props

@@ -51,7 +51,7 @@
 
   <Target Name="PrepareReleaseNotes" BeforeTargets="GenerateNuspec" DependsOnTargets="GetBuildVersion">
     <PropertyGroup>
-      <PackageReleaseNotes Condition="'$(PackageProjectUrl)'!=''">$(PackageProjectUrl)/releases/tag/v$(Version)</PackageReleaseNotes>
+      <PackageReleaseNotes Condition="'$(RepositoryUrl)'!=''">$(RepositoryUrl)/releases/tag/v$(Version)</PackageReleaseNotes>
     </PropertyGroup>
   </Target>
 </Project>