log: fix doc comment for Ldate
log: fix doc comment for Ldate

Fixes issue 3303.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/5795062
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go