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

skyportsystems/go.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,d0f/go-zh.tools,8limbs/gotools,bryanxu/go-zh.tools,8limbs/gotools,8limbs/gotools,webfd/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools