net/http/httputil: fix typo in comment.
net/http/httputil: fix typo in comment.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/6272050
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go