meetup link change

Changed the Meetup.com link.

The old <http://meetup.go-steel-programmers.org/> no longer ‛works’; it
used to redirect to <http://www.meetup.com/Go-Steel-Programmers/> even
after Meetup stopped supporting custom domains, but now it just
redirects to the front-page.  So change our link here.

I'm unhappy at the loss of flexibility, especially since they still
don't offer HTTPS.  We *should* be keeping all domain-names under our
control.

Will remove DNS shortly.