Update variant expected-result file.

This should have been updated in d2d8a229, but it was overlooked.
According to 31a877f1 which added it, this file is meant to show the
results you get under default_transaction_isolation = serializable.
We've largely lost track of that goal in other isolation tests, but
as long as we've got this one, it should be right.

Noted while fooling about with the isolationtester.

src/test/isolation/expected/drop-index-concurrently-1_2.out

@@ -21,7 +21,7 @@ QUERY PLAN
 
 Sort           
   Sort Key: id, data
-  ->  Seq Scan on test_dc
+  ->  Index Scan using test_dc_pkey on test_dc
         Filter: ((data)::text = '34'::text)
 step select2: SELECT * FROM test_dc WHERE data=34 ORDER BY id,data;
 id             data