Fix spelling errors in method names in QStorageInfo.

The methods retreiveVolumeInfo() and retreiveDiskFreeSpace() should be
retrieveVolumeInfo() and retrieveDiskFreeSpace().

Task-number: QTBUG-42068
Change-Id: I86137fbff26c42c8cc8bad366a4733118d6ca292
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>