Use QDir::homePath() insted of QDir::home().dirName() in doc example

In QInputDialog usage example (http://qt-project.org/doc/qt-5.1/qtwidgets/qinputdialog.html#details) used code:
QDir::home().dirName(), but QDir::homePath() is shorter and theoretically faster, because home() is just a wrapper:

static inline QDir home() { return QDir(homePath()); }