Fix incorrect function reference in comment of twophase.c

The header block of TwoPhaseGetDummyBackendId mentioned incorrectly
TwoPhaseGetDummyProc.

Reported-by: Oleksii Kliukin
Discussion: https://postgr.es/m/D8336E40-BBE1-4954-98BB-7830D3F5CB36@hintbits.com

src/backend/access/transam/twophase.c

@@ -844,7 +844,7 @@ TwoPhaseGetGXact(TransactionId xid)
 }
 
 /*
- * TwoPhaseGetDummyProc
+ * TwoPhaseGetDummyBackendId
  *		Get the dummy backend ID for prepared transaction specified by XID
  *
  * Dummy backend IDs are similar to real backend IDs of real backends.