Fix typo in standby.c

Hi,

I noticed a possible typo in standby.c:

---
 * The definitions of RunningTransactionsData and xl_xact_running_xacts are
 * similar. We keep them separate because xl_xact_running_xacts is a
---

It seems "xl_xact_running_xacts" should be "xl_running_xacts".

Best regards,
Hou zhijie