docs: Correct the release date of 5.2 stable

docs: Correct the release date of 5.2 stable

A table lists the 5.2 stable release date as September 15, but it was
released on July 7. This may confuse a reader who is trying to
understand the stable update release cycle.

Signed-off-by: Billy Wilson <billy_wilson@byu.edu>
Link: https://lore.kernel.org/r/20200806231754.7735-1-billy_wilson@byu.edu
Signed-off-by: Jonathan Corbet <corbet@lwn.net>