Correct AddOpenApi() service registration syntax (#36731)

* Initial plan

* Fix incorrect service registration syntax for AddOpenApi() calls

Co-authored-by: wadepickett <10985336+wadepickett@users.noreply.github.com>

* Fix typo in OpenAPI documents registration section

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: wadepickett <10985336+wadepickett@users.noreply.github.com>
Co-authored-by: Wade Pickett <wpickett@microsoft.com>

Author: Copilot

aspnetcore/fundamentals/openapi/using-openapi-documents.md

@@ -1,11 +1,12 @@
 ---
 title: Use the generated OpenAPI documents
+ai-usage: ai-assisted
 author: captainsafia
 description: Learn how to use OpenAPI documents in an ASP.NET Core app.
-ms.author: safia
 monikerRange: '>= aspnetcore-6.0'
+ms.author: safia
 ms.custom: mvc
-ms.date: 01/13/2026
+ms.date: 02/02/2026
 uid: fundamentals/openapi/using-openapi-documents
 ---
 # Use openAPI documents
@@ -129,11 +130,11 @@ public class CustomDocumentService(
 }
 ```
 
-Register the service in your DI container. Note that service key should match the document name passed to <xref:Microsoft.Extensions.DependencyInjection.OpenApiServiceCollectionExtensions.AddOpenApi%2A>:
+Register the service in your DI container. Note that the service key should match the document name passed to <xref:Microsoft.Extensions.DependencyInjection.OpenApiServiceCollectionExtensions.AddOpenApi%2A>:
 
 ```csharp
-builder.AddOpenApi(); // Adds "v1" by default
-builder.AddOpenApi("v2");
+builder.Services.AddOpenApi(); // Adds "v1" by default
+builder.Services.AddOpenApi("v2");
 builder.Services.AddScoped<CustomDocumentService>();
 ```