website: add disclaimer about AsiaBSDCon 2020

[0mp]

Summary

Both the main FreeBSD.org website, and the Events page, still show AsiaBSDCon 2020 as "upcoming". Unfortunately we need to note that it has been canceled.

I would do this commit myself but I do not understand how to build the website; I only understand the articles/books part.

[bcr]

Thanks for that change, it's good to make it. Here is how you can build the website: https://www.freebsd.org/doc/en_US.ISO8859-1/books/fdp-primer/the-website-build.html

I'm not sure if the paragraph you inserted will be shown as intended. If not, you can also just delete the entry.

[ygy]

@linimon I believe we can close this since the event is removed from the list.

[0mp]

I'm comandeering this revision to close it.