Update to do a unnecessary using

Co-authored-by: Vincent Biret <[email protected]>

Author: darrelmiller

src/Microsoft.OpenApi.Hidi/OpenApiService.cs

@@ -582,7 +582,7 @@ internal static async Task ShowOpenApiDocument(string openapi, FileInfo output,
                     // Create OpenApiUrlTree from document
 
                     using var file = new FileStream(output.FullName, FileMode.Create);
-                    var writer = new StreamWriter(file);
+                    using var writer = new StreamWriter(file);
                     WriteTreeDocument(openapi, document, writer);
                     writer.Flush();