CfC: Errata for correct use of compacted version of Public object

Issue #404 in the ActivityPub repository notes that AS2 requires 
compaction of AS2 objects. This will change the full URL for the Public 
collection, https://www.w3.org/ns/activitystreams#Public, to the 
compacted prefixed version as:Public. We use the longer version in a 
number of examples, which is technically incorrect.

https://github.com/w3c/activitypub/issues/404

To fix this error, after discussion in issue triage, I added the 
following proposed errata for the ActivityPub specification:

  * In Examples 6, 10, 15, 16 the public collection address should be
    compacted to "as:Public".

  * The Note in section 5.6 "Public Addressing" should read/Compacting
    an ActivityStreams object using the ActivityStreams JSON-LD context
    will result in "https://www.w3.org/ns/activitystreams#Public" being
    represented as "as:Public". Publishers might erroneously produce
    representations with the full URI or with the unprefixed value
    "Public". To maximize interoperability, consumers can accept all
    three representations./

This is a call for consensus on adding these errata to the main errata 
page. After 14 days, if there are no objections, the changes to the 
errata will be made, and additions to the editor's draft will also be added.

Evan