html: fix small typo
html: fix small typo

LGTM=dsymonds, adg, crawshaw, r, bradfitz, campoy, nigeltao, sameer, iant, robsc, djd, michael.j.fromberger, gmlewis, adonovan, rsc
R=golang-codereviews, dsymonds, adg, crawshaw, r, nigeltao, sameer, iant, robsc, djd, michael.j.fromberger, gmlewis, adonovan, rsc
CC=golang-codereviews
https://codereview.appspot.com/98880043

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang