Rename macro related to pg_backup_stop()
This should have been part of
39969e2 that has renamed pg_stop_backup()
to pg_backup_stop(), and this one is the last reference to
pg_stop/start_backup() I could find in the tree.
Author: Bharath Rupireddy
Discussion: https://postgr.es/m/CALj2ACXjvC28ppeDTCrfaSyHga0ggP5nRLJbsjx=7N-74UT4QA@mail.gmail.com