Update a file with file header to fix Build

- Previous Commit added support for Analyzer Fixer based file header
  addition to source code files. One was missing and wasn't updated
  through the script since the script only checks starting newline
  or a single comment line which were not enough to fix the file.

- Fortunately, the Analyzer caught this as IDE0073 style error and
  was able fix with a simple click on the suggestion icon in VS IDE.

NOTE: Previous Commit was supposed to fail to demonstrate
      build failure when using `file_header_template`.

Author: Nirmal4G

CommunityToolkit.Mvvm.SourceGenerators/Diagnostics/SuppressionDescriptors.cs

@@ -1,3 +1,7 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
 using Microsoft.CodeAnalysis;
 
 namespace CommunityToolkit.Mvvm.SourceGenerators.Diagnostics;