content: update references to code.google.com packages

Fixes golang/go#14182

Change-Id: Idaf4ef5a38b07fd100c7f869c07e680d148495b6
Reviewed-on: https://go-review.googlesource.com/19089
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
2 files changed