merge: clarifying two phase commit example

Author: Sam Kleinman

source/tutorial/perform-two-phase-commits.txt

@@ -229,8 +229,8 @@ to set remove the pending transaction from the :term:`documents
 
 .. code-block:: javascript
 
-   db.accounts.update({name: t.source}, {$pull: {pendingTransactions: ObjectId("4d7bc7a8b8a04f5126961522")}})
-   db.accounts.update({name: t.destination}, {$pull: {pendingTransactions: ObjectId("4d7bc7a8b8a04f5126961522")}})
+   db.accounts.update({name: t.source}, {$pull: {pendingTransactions: t._id}})
+   db.accounts.update({name: t.destination}, {$pull: {pendingTransactions: t._id}})
    db.accounts.find()
 
 The :method:`find() <db.collection.find()>` operation will return the