add missing []

BUG=skia:6039

NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=5653

Change-Id: I8bb0e0fec71cea99a9804cc855371fd153b52ce1
Reviewed-on: https://skia-review.googlesource.com/5653
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
1 file changed