doc/codewalk: update for changes in log package
doc/codewalk: update for changes in log package

R=adg, r
CC=golang-dev
http://codereview.appspot.com/2984041

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go