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