message: Fix QT6 example
diff: #include <string>
  +#include <string_view>
  #include <utility>
  #include "caf/all.hpp"
  @@ -74,7 +75,7 @@ void ChatWidget::sendChatMessage() {
  } else if (line.startsWith('/')) {
  vector<string> words;
  auto utf8 = QStringView{line}.mid(1).toUtf8();
  - auto sv = caf::string_view{utf8.constData(),
  + auto sv = std::string_view{utf8.constData(),
  static_cast<size_t>(utf8.size())};
  split(words, sv, is_any_of(" "));
  if (words.size() == 3 && words[0] == "join") {
  