Doc: corrected reference to customwidget.pro paragraph

Referred to par 1. Corrected to 3.

Task-number: QTBUG-29101
Change-Id: Icaa0e4eb06f24fcc7f2d369401fa8f49ed3b1d29
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>