Update QtQuick import value to 2.3

The current documentation uses import QtQuick 2.2 which is wrong since
there are new APIs like mipmap for the QML Image element.

Task-number: QTBUG-39302
Change-Id: I3cc3a1522efd85090c3b4011429fa16b95e79435
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>