add missing license header

Author: Mpdreamz

src/docs-assembler/Program.cs

@@ -1,4 +1,6 @@
-// See https://aka.ms/new-console-template for more information
+// Licensed to Elasticsearch B.V under one or more agreements.
+// Elasticsearch B.V licenses this file to you under the Apache 2.0 License.
+// See the LICENSE file in the project root for more information
 
 using System.Collections.Concurrent;
 using System.Diagnostics;