Fix incorrect CREATE STATISTICS example in docs

The example was incorrectly using parantheses around the list of columns, so
just drop them.

Reported-By: Robert Haas
Discussion: https://postgr.es/m/CA%2BTgmoZZEMAqWMAfvLHZnK57SoxOutgvE-ALO94WsRA7zZ7wyQ%40mail.gmail.com

doc/src/sgml/ref/create_statistics.sgml

@@ -178,7 +178,7 @@ CREATE TABLE t2 (
 INSERT INTO t2 SELECT mod(i,100), mod(i,100)
                  FROM generate_series(1,1000000) s(i);
 
-CREATE STATISTICS s2 (mcv) ON (a, b) FROM t2;
+CREATE STATISTICS s2 (mcv) ON a, b FROM t2;
 
 ANALYZE t2;