Fixed bug that slipped probably slipped in after rebasing and
explain why it failed on nacl/netbsd/plan9, which set default
maxparallelism to 1.

Change-Id: I4d59682fb2843d138b320334189f53fcdda5b2f6
Reviewed-on: https://go-review.googlesource.com/20980
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>