Fix typo in header comment

Commit 4c04be9b0 accidentally left off the _id portion of the function
name in the header comment.

Author: Junwang Zhao <zhjwpku@gmail.com>
Discussion: https://postgr.es/m/CAEG8a3LP+ytnAXSzR=yiEaQrde+iCybMHsuPn9n=UN3puV_1tw@mail.gmail.com

src/backend/utils/adt/xid8funcs.c

@@ -10,9 +10,9 @@
  * via functions such as SubTransGetTopmostTransaction().
  *
  * These functions are used to support the txid_XXX functions and the newer
- * pg_current_xact, pg_current_snapshot and related fmgr functions, since the
- * only difference between them is whether they expose xid8 or int8 values to
- * users.  The txid_XXX variants should eventually be dropped.
+ * pg_current_xact_id, pg_current_snapshot and related fmgr functions, since
+ * the only difference between them is whether they expose xid8 or int8 values
+ * to users.  The txid_XXX variants should eventually be dropped.
  *
  *
  *	Copyright (c) 2003-2023, PostgreSQL Global Development Group