fix instance name

Author: ldetmer

google-cloud-spanner/src/test/java/com/google/cloud/spanner/connection/it/ITMutableCredentialsTest.java

@@ -83,7 +83,7 @@ public void testMutableCredentialsUpdateAuthorizationForRunningClient() throws I
       InstanceName instanceName =
           InstanceName.of(
               getTestEnv().getTestHelper().getInstanceId().getProject(),
-              getTestEnv().getTestHelper().getInstanceId().getProject());
+              getTestEnv().getTestHelper().getInstanceId().getInstance());
       DatabaseAdminClient.ListDatabasesPagedResponse response =
           databaseAdminClient.listDatabases(instanceName);