incorrect docs for MessageRoutingCallback

[minhster99]

Describe the bug
ver 4.1.0

docs show the signature on the interface as
FunctionRoutingResult routingResult(Message<?> message);

https://docs.spring.io/spring-cloud-function/reference/spring-cloud-function/programming-model.html#messageroutingcallback

but looking at the interface, it is actually
String routingResult(Message<?> message)

https://github.com/spring-cloud/spring-cloud-function/blob/bd155628da87f2550469d8d1b7045c15430151e8/spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/MessageRoutingCallback.java#L47C1-L47C1

the javadocs for the interface is documented incorrectly as well and still refers to FunctionRoutingResult

Sample
n/a