=doc #16198 FSM extends Actor, not depends on it

The documentation incorrectly stated that FSM uses self type references
to depend on Actor, but it’s false, FSM directly extends Actor.
`akka-docs/rst/java/fsm.rst` doesn't speak of class hierarchies, and as far as I can judge, `akka-docs/rst/java/lambda-fsm.rst` is correct:

> The `AbstractFSM` abstract class is the base class used to implement an FSM. It implements Actor since an Actor is created to drive the FSM.

Maybe I could expand `java/fsm.rst`, but since it's a bit short-spoken I'm clueless where and what to add.