_content: fix StackOverflow link in the menu

The StackOverflow Go community is no longer active; replace by a link to
the [go] discussion tag.

Change-Id: I1435ff1df739717f30594e583412ecf587b848c2
Reviewed-on: https://go-review.googlesource.com/c/website/+/470815
Reviewed-by: Eli Bendersky <eliben@google.com>
Auto-Submit: Eli Bendersky‎ <eliben@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Eli Bendersky‎ <eliben@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

_content/menus.yaml

@@ -63,7 +63,7 @@ main:
             {img: slack.svg, url: 'https://invite.slack.golangbridge.org/'},
             {
               img: stack-overflow.svg,
-              url: 'https://stackoverflow.com/collectives/go',
+              url: 'https://stackoverflow.com/tags/go',
             },
           ]