go/types, types2: use go.dev/issue/nnnnn when referring to an issue (cleanup)

go/types, types2: use go.dev/issue/nnnnn when referring to an issue (cleanup)

Follow-up on CL 462856 which missed a few places.
Fixed manually.

Change-Id: I924560ecae8923d9228027016805a3cc892f8ac2
Reviewed-on: https://go-review.googlesource.com/c/go/+/463749
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>