Fix documentation of qmlvideofx example

Make the documentation fit to the current state of the example.

Task-number: QTBUG-38121
Change-Id: I3379e3328d07ef8bcdb35a5e3821ab194c3443c4
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Sami Makkonen <sami.makkonen@digia.com>
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>