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
https://golang.org/cl/4684044
1 file changed