Minor fix: Provide a string for "%s".

R=hablich@chromium.org

Change-Id: Ibae24688c5f69e0fee5108701aa7f483117aea8c
Reviewed-on: https://chromium-review.googlesource.com/457998Reviewed-by: Michael Hablich <hablich@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44350}