upgraded Example to new .csproj format

Example/Example.csproj

@@ -5,6 +5,10 @@
     <TargetFramework>netcoreapp3.1</TargetFramework>
   </PropertyGroup>
 
+  <ItemGroup>
+    <PackageReference Include="NStack.Core" Version="0.14.0" />
+  </ItemGroup>
+
   <ItemGroup>
     <ProjectReference Include="..\Terminal.Gui\Terminal.Gui.csproj" />
   </ItemGroup>