golang.org/cl/157097 modified this comment, but left a trailing comma.
While at it, make the sentence a bit clearer.

Change-Id: I376dda4fd18ddbcae4485dd660a79b9f66ad6da4
Reviewed-on: https://go-review.googlesource.com/c/158037Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>