spanner/apiv1: refer users to spanner client (#431)

diff --git a/google/spanner/v1/spanner_gapic.yaml b/google/spanner/v1/spanner_gapic.yaml
index 59e6914..2bcf25a 100644
--- a/google/spanner/v1/spanner_gapic.yaml
+++ b/google/spanner/v1/spanner_gapic.yaml
@@ -6,6 +6,7 @@
     package_name: google.cloud.spanner_v1.gapic
   go:
     package_name: cloud.google.com/go/spanner/apiv1
+    domain_layer_location: cloud.google.com/go/spanner
   csharp:
     package_name: Google.Cloud.Spanner.V1
   ruby: