doc/play: update obsolete comment in peano.go.

Change-Id: Ibecc2ef1e70c927c325c48bc381091f52c09da85
Reviewed-on: https://go-review.googlesource.com/1355
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>