[release-branch.go1.3] doc: drop scheme from links that are known to support HTTPS

««« CL 112650043 / bdac8d858e74
doc: drop scheme from links that are known to support HTTPS

golang.org now serves HTTPS with a valid cert, so it's reasonable
that users should click through to the HTTPS versions of *.golang.org
and other known sites.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/112650043
»»»

TBR=bradfitz
R=bradfitz
CC=golang-codereviews
https://codereview.appspot.com/111700043