doc/next: use relative link to Go issue

The Go release notes are served on multiple domains (go.dev,
golang.google.cn, tip.golang.org, localhost:8080 and so on), so links
pointing to the Go website itself need to be relative to work in all
those contexts.

Caught by a test in x/website. The next CL adds the same test to this
repository so these kinds of problems are caught sooner and with less
friction.

For #68545.
Fixes #68575.

Change-Id: I08056b98968c77a1d0ed93b63fccfbe41274ec8f
Reviewed-on: https://go-review.googlesource.com/c/go/+/600656
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: dmitshur

doc/next/3-tools.md

@@ -9,4 +9,4 @@ incompatible declarations. For instance, if `f` is declared as both `void f(int)
 and `void f(double)`, cgo will report an error instead of possibly generating an
 incorrect call sequence for `f(0)`. New in this release is a better detector for
 this error condition when the incompatible declarations appear in different
-files. See [#67699](https://go.dev/issue/67699).
+files. See [#67699](/issue/67699).