Use an underscore instead of a hyphen in the overview design doc’s
filename for consistency with the rest of the files in the repository.
Change-Id: I15a76a00709a43dfec60e7f4f6ee64a3cd031b2c
Reviewed-on: https://chromium-review.googlesource.com/458537
Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
