Change Mono#then to Mono#flatMap in documentation

Issue: SPR-15318

src/docs/asciidoc/web/web-flux-functional.adoc

@@ -94,7 +94,7 @@ public class PersonHandler {
 		Mono<ServerResponse> notFound = ServerResponse.notFound().build();
 		Mono<Person> personMono = this.repository.getPerson(personId);
 		return personMono
-				.then(person -> ServerResponse.ok().contentType(APPLICATION_JSON).body(fromObject(person)))
+				.flatMap(person -> ServerResponse.ok().contentType(APPLICATION_JSON).body(fromObject(person)))
 				.switchIfEmpty(notFound);
 	}
 }

src/docs/asciidoc/web/web-flux.adoc

@@ -150,7 +150,7 @@ Mono<Account> account = client.get()
 		.url("/accounts/{id}", 1L)
 		.accept(APPLICATION_JSON)
 		.exchange(request)
-		.then(response -> response.bodyToMono(Account.class));
+		.flatMap(response -> response.bodyToMono(Account.class));
 ----