chore: Update docs for Public Router NAT (#15765)

Author: gorge511

mmv1/products/compute/RouterNat.yaml

@@ -491,7 +491,7 @@ properties:
     type: Boolean
     description: |
       Enable endpoint independent mapping.
-      For more information see the [official documentation](https://cloud.google.com/nat/docs/overview#specs-rfcs).
+      For more information see the [official documentation](https://docs.cloud.google.com/nat/docs/public-nat#specs-rfcs).
     default_from_api: true
     send_empty_value: true
   - name: 'type'