Merge pull request #475 from w3c/issue-439

`summary` not allowed in `Mention` object. Closes #439.

Author: evanp

ERRATA.md

@@ -4,8 +4,18 @@ This document includes errata for the [Activity Streams](https://www.w3.org/TR/a
 
 ## Activity Streams
 
-- None yet reported.
+  - None yet reported.
 
 ## Activity Vocabulary
 
-- None yet reported.
+  - Example 58 includes a `summary` property on a `Mention` object, which is
+    not allowed. A corrected example:
+    
+    ```json
+    {
+      "@context": "https://www.w3.org/ns/activitystreams",
+      "type": "Mention",
+      "href": "http://example.org/joe",
+      "name": "Joe"
+    }
+    ```