Update comment

Function was renamed/replaced in
c2fe139c20 but the header comment was
not updated.

src/backend/access/index/indexam.c

@@ -26,7 +26,7 @@
  *		index_beginscan_parallel - join parallel index scan
  *		index_getnext_tid	- get the next TID from a scan
  *		index_fetch_heap		- get the scan's next heap tuple
- *		index_getnext	- get the next heap tuple from a scan
+ *		index_getnext_slot	- get the next tuple from a scan
  *		index_getbitmap - get all tuples from a scan
  *		index_bulk_delete	- bulk deletion of index tuples
  *		index_vacuum_cleanup	- post-deletion cleanup of an index