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
76def4cdd7 as a copy-pasto.

Backpatch-through: 18

src/backend/utils/activity/pgstat_backend.c

@@ -41,9 +41,9 @@ static bool backend_has_iostats = false;
 
 /*
  * WAL usage counters saved from pgWalUsage at the previous call to
- * pgstat_report_wal().  This is used to calculate how much WAL usage
- * happens between pgstat_report_wal() calls, by subtracting the previous
- * counters from the current ones.
+ * pgstat_flush_backend().  This is used to calculate how much WAL usage
+ * happens between pgstat_flush_backend() calls, by subtracting the
+ * previous counters from the current ones.
  */
 static WalUsage prevBackendWalUsage;