Add attribute namespace (#14975)

RequiredAttribute etc comes from the System.ComponentModel.DataAnnotations namespace.
Previously implied coming from Microsoft.Extensions.DependencyInjection

aspnetcore/fundamentals/configuration/options.md

@@ -339,6 +339,7 @@ public interface IValidateOptions<TOptions> where TOptions : class
 Data Annotation-based validation is available from the [Microsoft.Extensions.Options.DataAnnotations](https://www.nuget.org/packages/Microsoft.Extensions.Options.DataAnnotations) package by calling the <xref:Microsoft.Extensions.DependencyInjection.OptionsBuilderDataAnnotationsExtensions.ValidateDataAnnotations*> method on `OptionsBuilder<TOptions>`. `Microsoft.Extensions.Options.DataAnnotations` is included in the [Microsoft.AspNetCore.App metapackage](xref:fundamentals/metapackage-app) (ASP.NET Core 2.2 or later).
 
 ```csharp
+using System.ComponentModel.DataAnnotations;
 using Microsoft.Extensions.DependencyInjection;
 
 private class AnnotatedOptions