CFC: grammatical errors in definition of `tag` property

There are two grammatical errors in the definition of the `tag` property in
the Activity Vocabulary:

https://github.com/w3c/activitystreams/issues/624
https://github.com/w3c/activitystreams/issues/621

I've created a PR for the Activity Vocabulary errata to note this error and
propose one solution:

https://github.com/w3c/activitystreams/pull/626

This is a call for consensus. If there are no objections in the next 14
calendar days, I will merge the PR, and apply the solution to the editor's
draft of Activity Vocabulary.

Evan