Correct TrackingEvents parent: NonLinearAds, not NonLinear (fixes vast#55)

[aleksUIX]

Fixes InteractiveAdvertisingBureau/vast#55.

Problem

Section 3.14.2 in both 4.3.md and 4.4.md lists the parents of <TrackingEvents> as Linear, NonLinear, Companion. The NonLinear entry is incorrect: tracking for nonlinear creative hangs off the <NonLinearAds> container, not the individual <NonLinear> element. As the issue notes, this error carried over from the 4.3 PDF.

Evidence

• This document, section 3.12: the NonLinearAds feature table lists sub-elements NonLinear TrackingEvents, and its prose says the container holds "the <NonLinear> creative files and tracking resources". Section 3.12.1's NonLinear sub-element list contains no TrackingEvents.
• The published vast_4.2.xsd (in the vast repo): TrackingEvents children exist on Linear_Base_type, CompanionAd_type, and both the InLine and Wrapper NonLinearAds elements; the NonLinear type has none.
• VAST 2.0 and 3.0 placed TrackingEvents under NonLinearAds as well, so there is no version in which NonLinear was the parent.

I maintain a VAST validator (vastlint) whose structural model was built against the XSDs and this spec; this contradiction between 3.12 and 3.14.2 is one of the spots where the prose and the schema disagree and implementers guess.

Change

Four lines across 4.3.md and 4.4.md, section 3.14.2 only:

• Prose: "available for the Linear, NonLinearAds, and Companion elements" (also removes a stray comma in that sentence).
• Feature table: | Parent | Linear NonLinearAds Companion |

Section 3.14.3's note that tracking event values "differ for <Linear>, <NonLinear>, and <Companion>" is left untouched, since that sentence is about metric categories per creative type, not element parentage.