Docs: fix setters.yaml link in lz readme - during double PR merge sequence target v2 was moved/deprecated into the legacy folder

An inadvertent result of a sequence of PRs where one merged that moved files that were previously in another spot when the previous linked to them ok.

fix
https://github.com/GoogleCloudPlatform/pubsec-declarative-toolkit/tree/main/docs/landing-zone-v2#2-create-your-landing-zone
link
https://github.com/GoogleCloudPlatform/pubsec-declarative-toolkit/blob/main/solutions/landing-zone-v2/setters.yaml
to
https://github.com/GoogleCloudPlatform/pubsec-declarative-toolkit/blob/main/solutions/core-landing-zone/setters.yaml
dont need to reference where it actually is - as it is now deprecated in
https://github.com/GoogleCloudPlatform/pubsec-declarative-toolkit/blob/main/solutions/legacy/landing-zone-v2/setters.yaml

the following
https://github.com/GoogleCloudPlatform/pubsec-declarative-toolkit/pull/441
was affected by
https://github.com/GoogleCloudPlatform/pubsec-declarative-toolkit/pull/447

TODO: check all incoming links into files moved into the legacy folder for redirection
see one change in https://github.com/GoogleCloudPlatform/pubsec-declarative-toolkit/issues/475