Fix code comments still referring to pg_start/stop_backup()
pg_start_backup() and pg_stop_backup() have been respectively renamed to
pg_backup_start() and pg_backup_stop() as of
39969e2, but a few comments
did not get the call.
Reviewed-by: Kyotaro Horiguchi, David Steele
Discussion: https://postgr.es/m/YrqGlj1+4DF3dbZ/@paquier.xyz