Doc: Fix qRound64 Example documentation

Change Example code for qRound64.
qRound to qRound64 and int to qint64.

Task-number: QTBUG-39932
Change-Id: I6b423013ed539eaec396c84945e7a885b198aec4
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>