Correct QString::toStdString documentation on encoding

This statement was left from when toAscii was used, instead of toUtf8.
There is no loss of information when converting to UTF-8.

Change-Id: Iad92977af319b324cbf06f1a24712b31a7836622
Reviewed-by: Louai Al-Khanji <louai.al-khanji@digia.com>
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>