Spring Fix WebClientAdapter.createHttpServiceProxyFactory() example in ref docs

Following the changes applied to HttpServiceProxyFactory and WebClientAdapter in 48c1746 and b72ee5f, this PR applies fixes into HTTP Interface snippet in order to correctly initialize the client.

Comment From: sbrannen

Good catch.

This has been merged into main.

Thanks, and congratulations on submitting your first PR to the Spring Framework! 👍