Doc: Updating the Qt Serial Port URL.

When Qt 5.2 is released, it will be at
 http://qt-project.org/doc/qt-5.2/qtserialport-index.html

Change-Id: I2b81f8546e3df25c952eaa917954bc4a5100d6c9
Reviewed-by: Laszlo Papp <lpapp@kde.org>