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://golang.org/cl/100460047

evandbrown/build,evandbrown/build,evandbrown/build,golang/build,proppy/build,golang/build,proppy/build,golang/build,golang/build,proppy/build,proppy/build