content: fix links to code.google.com

Some minor fixes and clean up to articles that referred to
code.google.com (now defunct).

This cleans up all of the top 30 articles (not much point in cleaning
up the articles that are less viewed).

Fixes golang/go#18907.

Change-Id: I38a22f57e1effa8a545b429dface088042df0f91
Reviewed-on: https://go-review.googlesource.com/36311
Reviewed-by: Russ Cox <rsc@golang.org>
6 files changed