Doc: Fix some warnings in Qt Quick Controls

Set 'exampledirs' to '../../../examples/quick/controls' instead, so
the files under 'examples/quick/dialogs' are not processed twice.

Also fix a small typo in BusyIndicatorStyle.qml

Change-Id: Id5b3ff144056ab37e4a9e9f52cd1811797c54fb1
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>