Update source/tutorial/write-a-tumblelog-application-with-flask-mongoengine.txt

Updated app.config to reflect changes in flask-mongoengine; it now expects a "MONGODB_SETTINGS" dict (https://github.com/MongoEngine/flask-mongoengine/commit/352670e2ec80a2c2846a757214a0c3dfe0fa6eb4).

While there is a fallback, I suspect it may be broken, and the new way is just better.