Reflect media type changes in reference documentation

This commit mentions the `"application/graphql-response+json`" media
type in the reference documentation as the main media type used by the
server.

See gh-563

Author: bclozel

spring-graphql-docs/src/docs/asciidoc/index.adoc

@@ -43,14 +43,13 @@ Spring MVC and one for Spring WebFlux. Both handle requests asynchronously and h
 equivalent functionality, but rely on blocking vs non-blocking I/O respectively for
 writing the HTTP response.
 
-Requests must use HTTP POST with GraphQL request details included as JSON in the
-request body, as defined in the proposed
-https://github.com/graphql/graphql-over-http/blob/main/spec/GraphQLOverHTTP.md[GraphQL over HTTP]
-specification. Once the JSON body has been successfully decoded, the HTTP response
-status is always 200 (OK), and any errors from GraphQL request execution appear in the
-"errors" section of the GraphQL response. The default and preferred choice of media type is
-`"application/graphql+json"`, but `"application/json"` is also supported, as described in the
-specification.
+Requests must use HTTP POST with `"application/json"` as content type and GraphQL request details
+included as JSON in the request body, as defined in the proposed
+https://github.com/graphql/graphql-over-http/blob/main/spec/GraphQLOverHTTP.md[GraphQL over HTTP] specification.
+Once the JSON body has been successfully decoded, the HTTP response status is always 200 (OK),
+and any errors from GraphQL request execution appear in the "errors" section of the GraphQL response.
+The default and preferred choice of media type is `"application/graphql-response+json"`, but `"application/json"`
+is also supported, as described in the specification.
 
 `GraphQlHttpHandler` can be exposed as an HTTP endpoint by declaring a `RouterFunction`
 bean and using the `RouterFunctions` from Spring MVC or WebFlux to create the route. The