People viewing this locally will not have a /s/ on their local godoc.
tip.golang.org doesn't have one either.

Also change all golang.org links to https, to avoid mixed content
warnings when viewing https://golang.org/.

Fixes #9028.

LGTM=bradfitz, r
R=r, bradfitz
CC=adg, golang-codereviews
https://golang.org/cl/168250043