Fix typos in standby.c

xl_running_xacts exists, not xl_xact_running_xacts.

Author: Hou Zhijie
Discussion: https://postgr.es/m/OS0PR01MB57160D8B01097FFB5C175065941A9@OS0PR01MB5716.jpnprd01.prod.outlook.com

src/backend/storage/ipc/standby.c

@@ -1273,10 +1273,10 @@ LogStandbySnapshot(void)
 /*
  * Record an enhanced snapshot of running transactions into WAL.
  *
- * The definitions of RunningTransactionsData and xl_xact_running_xacts are
- * similar. We keep them separate because xl_xact_running_xacts is a
- * contiguous chunk of memory and never exists fully until it is assembled in
- * WAL. The inserted records are marked as not being important for durability,
+ * The definitions of RunningTransactionsData and xl_running_xacts are
+ * similar. We keep them separate because xl_running_xacts is a contiguous
+ * chunk of memory and never exists fully until it is assembled in WAL.
+ * The inserted records are marked as not being important for durability,
  * to avoid triggering superfluous checkpoint / archiving activity.
  */
 static XLogRecPtr