doc/next: fix link

doc/next: fix link

It was rendering incorrectly at https://tip.golang.org/doc/go1.25

Change-Id: I2f66c95414ac5d71b9b02b91bcdc0d0a87b3f605
Reviewed-on: https://go-review.googlesource.com/c/go/+/662436
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>