Point to downloads.apache.org instead of www.apache.org/dist

Ref e-mail from INFRA today.

As of March 2020, we are deprecating www.apache.org/dist/ in favor of
https://downloads.apache.org/ for backup downloads as well as signature
and checksum verification. The primary driver has been splitting up web
site visits and downloads to gain better control and offer a better
service for both downloads and web site visits.