CfC: fix grammatical error in the description of the "audience" property

Per issue 600 <https://github.com/w3c/activitystreams/issues/600>, there 
is a grammatical issue in the description of the "audience" property. 
There is an erratum for changing "can considered" to "can be considered".

https://github.com/w3c/activitystreams/pull/601

This is a call for consensus. If there are no objections on this mailing 
list or in the comments of that issue or PR, I'll merge the PR to main, 
and update the editor's draft of AS2 Vocabulary.

Evan