message: Fix documentation text for remove_actor
diff: @@ -40,7 +40,7 @@ public:
  virtual void add_actor(const local_actor& self, const local_actor* parent)
  = 0;
  - /// Called whenever the actor system spawns a new actor.
  + /// Called before the actor system calls the destructor for `self`.
  /// @param ptr Points to an actor that is about to get destroyed.
  /// @thread-safe
  virtual void remove_actor(const local_actor& self) = 0;
  