Update app.py

app.py

@@ -331,7 +331,7 @@ def merge_trans():
             'amount': previous_post_rec['transactions'][0]['amount'],
             'balance': previous_post_rec['balance'], 
             'proof': previous_post_rec['proof'],
-            'previous_hash': previous_post['previous_hash']}
+            'previous_hash': previous_post_rec['previous_hash']}
     trans_bx.append(send_block)
     trans_bx.append(rec_block)