Dark Mode: fix paper-button shadow Y offset

Namrata made a small typo (3->2) in the spec.

TBR=dpapad@chromium.org
BUG=883049

Change-Id: I3c19ede9b2d9cfd02b2a5c0a308bf997388eb0b8
Reviewed-on: https://chromium-review.googlesource.com/c/1426058
Reviewed-by: Dan Beam <dbeam@chromium.org>
Commit-Queue: Dan Beam <dbeam@chromium.org>
Auto-Submit: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624688}
3 files changed