Fix comment in tableam.h about GetHeapamTableAmRoutine()

This routine is located in heapam_handler.c, not tableamapi.c.  Issue
noted while hacking the area for a different patch.

Reviewed-by: Richard Guo
Discussion: https://postgr.es/m/ZWQuHltp2KS_0Cct@paquier.xyz

Author: michaelpq

src/include/access/tableam.h

@@ -2095,6 +2095,12 @@ extern void table_block_relation_estimate_size(Relation rel,
  */
 
 extern const TableAmRoutine *GetTableAmRoutine(Oid amhandler);
+
+/* ----------------------------------------------------------------------------
+ * Functions in heapam_handler.c
+ * ----------------------------------------------------------------------------
+ */
+
 extern const TableAmRoutine *GetHeapamTableAmRoutine(void);
 
 #endif							/* TABLEAM_H */