Put attendant thread in background

This allow main thread to terminate without any error when each
car thread has terminated.
It's more accurate, because otherwise the attendant will wait
forever and block the program.

main.cpp

@@ -74,7 +74,10 @@ void attendant()
 int main()
 {
     print("Car wash opening with " << N << " waiting spaces");
+
     thread attendantThread(attendant);
+    attendantThread.detach();
+
     vector<thread> cars {};
     for(size_t i = 0; i < N+1; ++i)
     {
@@ -82,7 +85,6 @@ int main()
         cars.push_back(std::move(t));
     }
 
-    attendantThread.join();
     for (auto &car: cars) {
         car.join();
     }