Fix bogus comments

Some comments mentioned XLogReplayBuffer, but there's no such function:
that was an interim name for a function that got renamed to
XLogReadBufferForRedo, before commit 2c03216d83 was pushed.

src/backend/access/heap/heapam.c

@@ -7898,8 +7898,8 @@ heap_xlog_visible(XLogReaderState *record)
 			PageInit(vmpage, BLCKSZ, 0);
 
 		/*
-		 * XLogReplayBufferExtended locked the buffer. But visibilitymap_set
-		 * will handle locking itself.
+		 * XLogReadBufferForRedoExtended locked the buffer. But
+		 * visibilitymap_set will handle locking itself.
 		 */
 		LockBuffer(vmbuffer, BUFFER_LOCK_UNLOCK);
 

src/backend/access/transam/xlogutils.c

@@ -426,9 +426,10 @@ XLogReadBufferForRedoExtended(XLogReaderState *record,
  * to imply that the page should be dropped or truncated later.
  *
  * NB: A redo function should normally not call this directly. To get a page
- * to modify, use XLogReplayBuffer instead. It is important that all pages
- * modified by a WAL record are registered in the WAL records, or they will be
- * invisible to tools that that need to know which pages are modified.
+ * to modify, use XLogReadBufferForRedoExtended instead. It is important that
+ * all pages modified by a WAL record are registered in the WAL records, or
+ * they will be invisible to tools that that need to know which pages are
+ * modified.
  */
 Buffer
 XLogReadBufferExtended(RelFileNode rnode, ForkNumber forknum,