Fix System.Web Adapters service registration syntax in documentation

[VedikaGupt]

PR Description:
This PR corrects the syntax for registering System.Web Adapters services in the HttpApplicationHost.RegisterHost context. The AddSystemWebAdapters() method is an extension on IServiceCollection, so it should be called as builder.Services.AddSystemWebAdapters() instead of builder.AddSystemWebAdapters().

Fixes: #36520

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aspnetcore/migration/fx-to-core/inc/remote-app-setup.md	Remote app setup

[VedikaGupt]

@dotnet-policy-service agree company="Microsoft"

[CZEMacLeod]

I'm not sure this is correct -

public static ISystemWebAdapterBuilder AddSystemWebAdapters(this IHostApplicationBuilder builder)

Also the method uses builder.Configuration as well as builder.Services, so it needs access to both, at least on version 2.2.1.
This change also contradicts the readme in the package.

https://github.com/dotnet/systemweb-adapters/blob/main/src/Microsoft.AspNetCore.SystemWebAdapters.FrameworkServices/README.md

[VedikaGupt]

Thanks for pointing that out @CZEMacLeod!!