Issue #15228 describes that reserved address blocks should be used for
documentation purposes. This change updates the prefix length so the
IPv4 address adheres to this.

Change-Id: I237d9cce1a71f4fd95f927ec894ce53fa806047f
Reviewed-on: https://go-review.googlesource.com/42991
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>