CfC: Add AS2 context to Example 60

Per issue#441 <https://github.com/w3c/activitystreams/issues/441>, 
example 60 in the Activity Streams Vocabulary specification is missing 
its AS2 context. This change adds an erratum to correct this mistake. 
Once accepted, we can also merge to the editor's version of the spec itself.

https://github.com/w3c/activitystreams/pull/643

This is a call for consensus on this erratum. After 14 days, I'll merge 
this PR and update the ERRATA.md for Activity Streams. I'll also make 
the change in the editor's draft of AS2 Vocabulary.

Evan