Fix port script

We changed _simple_update_one_txn to use _simple_update_txn but didn't
yank it out in the port script.

Fixes #2565

scripts/synapse_port_db

@@ -112,6 +112,7 @@ class Store(object):
 
     _simple_update_one = SQLBaseStore.__dict__["_simple_update_one"]
     _simple_update_one_txn = SQLBaseStore.__dict__["_simple_update_one_txn"]
+    _simple_update_txn = SQLBaseStore.__dict__["_simple_update_txn"]
 
     def runInteraction(self, desc, func, *args, **kwargs):
         def r(conn):