Doc: Example code snippet for QtConcurrent::mapped does not compile

According to a discussion on Freenode/#qt-labs on 09042018 1428 CEST:

• http://doc.qt.io/qt-5/qtconcurrentmap.html <- the lambda example does not compile

• the error is it can't convert QFuture<void> to QFuture<QImage> or whatever result_type is

• it only works if you wrap the lambda into a std::function<T(T)>

Known issue:

• https://bugreports.qt.io/browse/QTBUG-33735
  QtConcurrent::mapped doesn't work with C++11 lambdas

even with the bug being open for 5 years, at least the docs should have working
examples