Fix typos in Spring MVC refdoc
Fix typos in Spring MVC refdoc

1. Consistent with "xml code" examples.
2. "xml()" is a static method and will use default builder config,
   so use createXmlMapper(true) instead.
3. Fix mvc namespace tag typo.

spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework