html: fix typo in package docs.
html: fix typo in package docs.

Fixes issue 2419.

R=dsymonds, rsc
CC=golang-dev
http://codereview.appspot.com/5352046
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go