adjust dblink regression expected output for commit 5da9868ed9

Seems the -1/singular output is used in the dblink regression tests.

Reported-by: Álvaro Herrera

Discussion: https://postgr.es/m/20210330231506.GA10666@alvherre.pgsql

contrib/dblink/expected/dblink.out

@@ -1099,9 +1099,9 @@ SELECT *
 FROM dblink('myconn',
     'SELECT * FROM (VALUES (''-1 2:03:04'')) i')
   AS i(i interval);
-         i         
--------------------
- -1 days -02:03:04
+        i         
+------------------
+ -1 day -02:03:04
 (1 row)
 
 -- Try swapping to another format to ensure the GUCs are tracked