doc/go1.18: point to spec in same directory for release notes

The release notes explicitly refer to sections updated
for generics in the spec but then point to the old spec
which is very confusing for beta users.

For #47694

Change-Id: I5b555db3543cc32f088a8b267ec3f1195a52a812
Reviewed-on: https://go-review.googlesource.com/c/go/+/373174


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>