Fixes go lint warning.

Change-Id: I63950e7c70bf431e88a04f32befd50be9beacadf
Reviewed-on: https://go-review.googlesource.com/108815Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>