http: update triv.go with gofix
http: update triv.go with gofix

Sample code in triv.go wouldn't compile because it didn't use the new
FileSystem interface.
This was discussed on golang-nuts on July 7.
https://groups.google.com/forum/#!topic/golang-nuts/NMhZk3Ft_Vc

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/4684044

Committer: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go