Fix outdated link to buffer article in README.rst

[radeklat]

Description

Fix outdated link to buffer article in the README.rst file.

Motivation and Context

The current link is broken and no longer works.

Have you tested this? If so, how?

The slug part of the URL is the same and the article mentions Luigi. So I assume it is the same.

[dlstadther]

Not sure why some of the ci tests failed. I've triggered them to rerun to see if the issues were transient.