SpringBoot Update RestTemplateBuilder#defaultHeader javadoc to reference correct client-side HTTP request class

The Javadoc of org.springframework.boot.web.client.RestTemplateBuilder#defaultHeader incorrectly references reactor.netty.http.client.HttpClientRequest. This is now replaced with the correct org.springframework.http.client.ClientHttpRequest.

Comment From: wilkinsona

@rob-valor Thanks very much for making your first contribution to Spring Boot.