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 referenced the
latter, but it is perfectly OK to flush the backend stats independently
of the WAL stats.

Noticed while looking at this area of the code, introduced by
76def4cdd7c2 as a copy-pasto.

Backpatch-through: 18