Correct minor error in INTERVAL_EVENT.width documentation

Andrew Patterson: The width attribute is defined as a DV_DURATION type and it is required. However the text says that it should be Void if the Interval_Event is instantaneous. There is also an invariant preventing width from being Void.

Tim Cook: I always read this to mean "where the event is instantaneous (i.e a POINT_EVENT) the duration is void". So I would contend that an INTERVAL_EVENT cannot be instantaneous, rather than allowing the INTERVAL_EVENT duration to be 0.

I agree that the wording could be better to clarify which interpretation is right..