Spring Fix typo in reference docs regarding RequestMappingHandlerAdapter

This PR fix a typo in referece docs.

A list of HttpMessageConverter's are registered as this.messageConverters in the constructor of RequestMappingHandlerAdapter (not RequestMethodHandlerAdapter). https://github.com/spring-projects/spring-framework/blob/be7fa3aaa879cfb44fc976be7331caf5479a943f/spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/RequestMappingHandlerAdapter.java#L203-L216

Comment From: sbrannen

Good catch!

This has been merged into 5.3.x and main.

Thanks