chore(spanner): update instance name

Author: harshachinta

.kokoro/build.sh

@@ -130,7 +130,7 @@ integration-multiplexed-sessions-enabled)
       -Dmaven.main.skip=true \
       -Dspanner.gce.config.server_url=https://staging-wrenchworks.sandbox.googleapis.com \
       -Dspanner.gce.config.project_id=span-cloud-testing \
-      -Dspanner.testenv.instance=projects/span-cloud-testing/instances/java-client-integration-tests-multiplexed-sessions \
+      -Dspanner.testenv.instance=projects/span-cloud-testing/instances/spanner-java-client-testing \
       -fae \
       verify
     RETURN_CODE=$?