Doc: add missing parameter name to function signature in docs

...to fix a QDoc error.

Change-Id: Id074b1e43fd1739a6dddd77b922b2483d0879727
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>