content, template: use https instead of protocol-relative URLs

Protocol-relative URLs (//blog.golang.org/...) are redundant, we use
HTTPS everywhere.
Use the https:// scheme instead.

Updates golang/go#17961.

Change-Id: I6be6dd1d85f1093561ce3fa606e1ef37d75a7a60
Reviewed-on: https://go-review.googlesource.com/37883
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed