go.tools/dashboard: fix printf buf found by go vet
go.tools/dashboard: fix printf buf found by go vet

LGTM=iant
R=adg, iant
CC=golang-codereviews
https://codereview.appspot.com/100460047

t-yuki/go.tools.old,t-yuki/go.tools.old,t-yuki/go.tools.old