CfC: Example 157 in Activity Streams Vocabulary is not valid JSON

Issue #448 in the Activity Streams repo notes that Example 157 is not 
valid JSON; a string value is shown as continuing across multiple lines.

https://github.com/w3c/activitystreams/issues/448

This pull request adds an erratum for Activity Streams that provides a 
corrected version:

https://github.com/w3c/activitystreams/pull/587

Text of the erratum:

    - Example 157is not valid JSON because the `content` property
    extends over multiple lines. The correct example is:
    ```json
    {
    "@context": "https://www.w3.org/ns/activitystreams",
    "name": "A thank-you note",
    "type": "Note",
    "content": "Thank you <a href='http://sally.example.org'>@sally</a>
    for all your hard work! <a
    href='http://example.org/tags/givingthanks'>#givingthanks</a>",
    "to": {
    "name": "Sally",
    "type": "Person",
    "id": "http://sally.example.org"
    },
    "tag": {
    "id": "http://example.org/tags/givingthanks",
    "name": "#givingthanks"
    }
    }
    ```

This is a call for consensus on these changes. If there are no 
objections to this solution within 14 days, I will merge this PR and 
update the example in the editor's draft.

Evan