Fix typo in comment.

Amit Langote

src/backend/access/heap/heapam.c

@@ -6547,7 +6547,7 @@ heap_prepare_freeze_tuple(HeapTupleHeader tuple, TransactionId cutoff_xid,
  *
  * Caller is responsible for ensuring that no other backend can access the
  * storage underlying this tuple, either by holding an exclusive lock on the
- * buffer containing it (which is what lazy VACUUM does), or by having it by
+ * buffer containing it (which is what lazy VACUUM does), or by having it be
  * in private storage (which is what CLUSTER and friends do).
  *
  * Note: it might seem we could make the changes without exclusive lock, since