Fix CREATE INDEX CONCURRENTLY example

It fails to use the CONCURRENTLY keyword where it was necessary, so add
it.  This text was added to pg11 in commit 5efd604ec0a3; backpatch to pg12.

Author: Nikolay Samokhvalov <nik@postgres.ai>
Discussion: https://postgr.es/m/CAM527d9iz6+=_c7EqSKaGzjqWvSeCeRVVvHZ1v3gDgjTtvgsbw@mail.gmail.com

doc/src/sgml/ddl.sgml

@@ -4175,7 +4175,7 @@ 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;