Improve grammar of message about two-phase state files.

When there's only one two-phase state file, there's also only one
long-running prepared transaction.  Adjust the message text
accordingly.

Nikhil Sontakke

Discussion: http://postgr.es/m/CAMGcDxcmR_DWZXXndGoPzVQx=B17A5=RviEA1qNaF=FWLy5Whw@mail.gmail.com

src/backend/access/transam/twophase.c

@@ -1647,7 +1647,7 @@ CheckPointTwoPhase(XLogRecPtr redo_horizon)
 	if (log_checkpoints && serialized_xacts > 0)
 		ereport(LOG,
 				(errmsg_plural("%u two-phase state file was written "
-							   "for long-running prepared transactions",
+							   "for a long-running prepared transaction",
 							   "%u two-phase state files were written "
 							   "for long-running prepared transactions",
 							   serialized_xacts,