message: Fix typo, relates
diff: @@ -548,7 +548,7 @@ public:
  CAF_HAS_MEMBER_TRAIT(size);
  CAF_HAS_MEMBER_TRAIT(data);
  -/// Checks whether T is convertible to either `std::function<void (T&)>`
  +/// Checks whether F is convertible to either `std::function<void (T&)>`
  /// or `std::function<void (const T&)>`.
  template <class F, class T>
  struct is_handler_for {
  