misc/makerelease/windows: fix 404 help URL in installer

ARPHELPLINK yields 404; update the URL.

While here, also prefix the ARPREADME and ARPURLINFOABOUT URL's with the HTTP scheme to make 'em clickable links in the Add or Remove Programs listing.

LGTM=adg
R=golang-codereviews
CC=adg, golang-codereviews
https://codereview.appspot.com/154580045

Committer: Andrew Gerrand <adg@golang.org>