pgsql: Fix incorrect comment in pgstat_backend.c

Fix incorrect comment in pgstat_backend.c The counters saved from pgWalUsage, used for the difference calculations when flushing the backend WAL stats, are updated when calling pgstat_flush_backend() under PGSTAT_BACKEND_FLUSH_WAL, and not pgstat_report_wal(). The comment updated in this commit r

pgsql: Fix incorrect comment in pgstat_backend.c

Fix incorrect comment in pgstat_backend.c The counters saved from pgWalUsage, used for the difference calculations when flushing the backend WAL stats, are updated when calling pgstat_flush_backend() under PGSTAT_BACKEND_FLUSH_WAL, and not pgstat_report_wal(). The comment updated in this commit r