golang.org/cl/147117 increased the number of arguments permitted
by Proc.Call on Windows, but the doc comment was never updated.

Change-Id: Iea5eb9e0aafbc1025d5fcb8665d028b2254c183a
Reviewed-on: https://go-review.googlesource.com/c/153825Reviewed-by: Channing Kimble-Brown <ckimblebrown@google.com>
Reviewed-by: Andrew Bonventre <andybons@golang.org>