Spring Fix explanation on default settings for content negotiation in reference doc

The official documentation "Web on Servlet Stack" contains paragraph "1.11.6. Content Types" that contains incorrect description of default strategy to determine the requested media types from the request. According to source code of "org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport" the PathExtensionContentNegotiationStrategy is deprecated and is not used as a default strategy.