CONCURRENTLY in example of index of partitioned table

[Nikolay Samokhvalov]

https://www.postgresql.org/docs/current/ddl-partitioning.html provides an
example of using CONCURRENTLY for index creation of partitioned tables; but
it the example itself doesn't have this word; here is a fix:

diff --git a/doc/src/sgml/ddl.sgml b/doc/src/sgml/ddl.sgml
index
4490e82aa52f3c0e2039047ab57194933d7c0275..d2951cd754c68237cd97eedd9c4353a671ad0995
100644
--- a/doc/src/sgml/ddl.sgml
+++ b/doc/src/sgml/ddl.sgml
@@ -4222,7 +4222,7 @@ ALTER TABLE measurement ATTACH PARTITION
measurement_y2008m02
 <programlisting>
 CREATE INDEX measurement_usls_idx ON ONLY measurement (unitsales);

-CREATE INDEX measurement_usls_200602_idx
+CREATE INDEX CONCURRENTLY measurement_usls_200602_idx
     ON measurement_y2006m02 (unitsales);
 ALTER INDEX measurement_usls_idx
     ATTACH PARTITION measurement_usls_200602_idx;