Update outdated comment for TransactionIdSetTreeStatus

Commit 06da3c570f changed the way subtransactions are marked as
SUBCOMMITTED, but the example it included actually documented the old
way. Update it.

Author: Japin Li
Discussion: https://www.postgresql.org/message-id/MEYP282MB16690BC96DFBE08CC857E1E3B6319%40MEYP282MB1669.AUSP282.PROD.OUTLOOK.COM

src/backend/access/transam/clog.c

@@ -146,9 +146,7 @@ static void TransactionIdSetPageStatusInternal(TransactionId xid, int nsubxids,
  *					page2: set t2,t3 as sub-committed
  *					page3: set t4 as sub-committed
  *		2. update page1:
- *					set t1 as sub-committed,
- *					then set t as committed,
-					then set t1 as committed
+ *					page1: set t,t1 as committed
  *		3. update pages2-3:
  *					page2: set t2,t3 as committed
  *					page3: set t4 as committed