Adjust macro name in pg_backup_stop()

Hi, it looks like the commit [1] renamed pg_stop_backup() to
pg_backup_stop() but forgot to rename the associated
PG_STOP_BACKUP_V2_COLS macro. While this is harmless, here's a patch
to rename the macro to be in sync with the function name.

Thoughts?

[1]
commit 39969e2a1e4d7f5a37f3ef37d53bbfe171e7d77a
Author: Stephen Frost <sfrost@snowman.net>
Date:   Wed Apr 6 14:41:03 2022 -0400

    Remove exclusive backup mode

-- 
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com