Fix typo in test comment.

src/test/regress/expected/stats_ext.out

@@ -272,7 +272,7 @@ SELECT s.stxkind, d.stxdndistinct
  {d,f,m} | {"3, 4": 2550, "3, 6": 800, "4, 6": 1632, "3, 4, 6": 5000}
 (1 row)
 
--- correct esimates
+-- correct estimates
 SELECT * FROM check_estimated_rows('SELECT COUNT(*) FROM ndistinct GROUP BY a, b');
  estimated | actual 
 -----------+--------

src/test/regress/sql/stats_ext.sql

@@ -188,7 +188,7 @@ SELECT s.stxkind, d.stxdndistinct
  WHERE s.stxrelid = 'ndistinct'::regclass
    AND d.stxoid = s.oid;
 
--- correct esimates
+-- correct estimates
 SELECT * FROM check_estimated_rows('SELECT COUNT(*) FROM ndistinct GROUP BY a, b');
 
 SELECT * FROM check_estimated_rows('SELECT COUNT(*) FROM ndistinct GROUP BY a, b, c');