[3.13] gh-141412: Use reliable target URL for urllib example (GH-141475)

The endpoint used for demonstrating reading URLs is no longer
stable. This change substitutes a target over which we have more
control.
(cherry picked from commit fbcac799518e0cb29fcf5f84ed1fa001010b9073)

Co-authored-by: Bob Kline <bkline@users.noreply.github.com>