CfC: Add terminating "Z" in example with a "startTime" property

The|startTime|property in Example 146 of Activity Streams 2.0 Vocabulary 
is missing a terminating Z. The Z indicates UTC time and is required for 
AS2, see https://www.w3.org/TR/activitystreams-core/#dates.


https://github.com/w3c/activitystreams/pull/590

This is a call for consensus. If there are no objections after 14 days, 
I'll apply this pull request to add an erratum. Once it's completed, 
I'll update the editor's draft of the AS2 Vocabulary document.


Evan