cmd/gopherbot: fix Printf vet failure
cmd/gopherbot: fix Printf vet failure

Currently "go test" on tip fails because there's no argument passed
for Printf.

Change-Id: I66a028bcb0f10f106c0dcc9643ba878363521127
Reviewed-on: https://go-review.googlesource.com/109458
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build