BUG #17446: Suggest updating the section title "Building Indexes Concurrently"

The following bug has been logged on the website:

Bug reference:      17446
Logged by:          Troy Frericks
Email address:      troy@frericks.us
PostgreSQL version: 10.17
Operating system:   Documentation suggestion, ambiguous
Description:

https://www.postgresql.org/docs/9.6/sql-createindex.html#SQL-CREATEINDEX-CONCURRENTLY

Suggest changing section title "Building Indexes Concurrently" to "Building
Indexes Using the CONCURRENTLY Parameter" to eliminate any ambiguity.