Fixes typo FunctionRoutingResult to String

docs/modules/ROOT/pages/spring-cloud-function/programming-model.adoc

@@ -195,8 +195,9 @@ The `MessageRoutingCallback` is a strategy to assist with determining the name o
 [source, java]
 ----
 public interface MessageRoutingCallback {
-	FunctionRoutingResult routingResult(Message<?> message);
-	. . .
+    default String routingResult(Message<?> message) {
+        return (String) message.getHeaders().get("spring.cloud.function.definition");
+    }
 }
 ----
 
@@ -739,4 +740,4 @@ private final static JsonMasker masker = JsonMasker.INSTANCE();
 . . .
 
 logger.info("Received: " + masker.mask(new String(payload, StandardCharsets.UTF_8)));
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+----