Fix code bug in Chapter 52 of User Guide

Reported by Petr Beran:

In chapter 51.2. RESTEasy Proxy Framework ( https://docs.jboss.org/resteasy/docs/4.6.0.Final/userguide/html/RESTEasy_Client_Framework.html#proxies ) there are 3 code snippets - first one with SimpleClient interface and the other two with the proxy itself.

In those last two snippets, interface method putBasic is called on ResteasyClient (client.putBasic("hello world"). But this method should be visible only from proxy SimpleClient simple, and as such the line should be simple.putBasic("Hello there");.