Update release notes link to use aka.ms. (dotnet#294)

Our release notes link is broken because the `Documentation` was renamed to `docs`. Fix this for the future to use a redirection link.

Author: eerhardt

pkg/Directory.Build.props

@@ -24,7 +24,7 @@
     <PackageLicenseUrl>https://github.com/dotnet/machinelearning/blob/master/LICENSE</PackageLicenseUrl>
     <PackageProjectUrl>https://dot.net/ml</PackageProjectUrl>
     <PackageIconUrl>https://aka.ms/mlnetlogo</PackageIconUrl>
-    <PackageReleaseNotes>https://github.com/dotnet/machinelearning/tree/master/Documentation/release-notes</PackageReleaseNotes>
+    <PackageReleaseNotes>https://aka.ms/mlnetreleasenotes</PackageReleaseNotes>
   </PropertyGroup>
 
   <ItemGroup>