_content: update footer menus link to go.dev/blog

Change-Id: I21804defcbcafc84de4cbd6b34758e4e4734b8ee
Reviewed-on: https://go-review.googlesource.com/c/website/+/369477
Run-TryBot: Jamal Carvalho <jamal@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

_content/menus.yaml

@@ -51,7 +51,7 @@ footer:
     - name: Release Notes
       url: /doc/devel/release
     - name: Brand Guidelines
-      url: https://blog.golang.org/go-brand
+      url: /blog/go-brand
     - name: Code of Conduct
       url: /conduct