chore: moves using after the copyright

Signed-off-by: Vincent Biret <vibiret@microsoft.com>

Author: baywet

test/Microsoft.OpenApi.Readers.Tests/V2Tests/OpenApiHeaderTests.cs

@@ -1,7 +1,7 @@
-using System.Text.Json.Nodes;
-// Copyright (c) Microsoft Corporation. All rights reserved.
+// Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license.
 
+using System.Text.Json.Nodes;
 using System.IO;
 using FluentAssertions;
 using FluentAssertions.Equivalency;