Simplify sample code

samples/SampleHelper.cs

@@ -0,0 +1,10 @@
+using System.Linq;
+using System.Reflection;
+
+internal static class SampleHelper
+{
+    public static string GetReleasesDir() => Assembly.GetEntryAssembly()
+        .GetCustomAttributes<AssemblyMetadataAttribute>()
+        .Where(x => x.Key == "VelopackSampleReleaseDir")
+        .Single().Value;
+}