Merge pull request #474 from w3c/issue-447

Leading space in `units` enum. Closes #447

Author: evanp

ERRATA.md

@@ -8,6 +8,13 @@ This document includes errata for the [Activity Streams](https://www.w3.org/TR/a
 
 ## Activity Vocabulary
 
+  - The range of the `units` property is given as an enumerated set of values.
+    Due to a formatting error, some of these values are shown with an incorrect
+    leading space character. The correct range is:
+    
+    ```text
+    "cm" | "feet" | "inches" | "km" | "m" | "miles" | xsd:anyURI
+    
   - Example 58 includes a `summary` property on a `Mention` object, which is
     not allowed. A corrected example: