Doc: Small fix for SplitView

SplitView items must set width/height and not
Layout.preferredWidth/Layout.preferredHeight.

Change-Id: Ice888d51c63cbdaf6e0e235c0c7569c4a8794b7d
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>