Update Connect to Mongo DB exercises

Some people might get hung up on this - posting here, as it might be good to update the lesson text.

specifically this is when you’re trying to connect to mongoose.connect(process.env.URI);

if you do that, you’ll get the following in the console… which could be easily missed by a learner.

(node:3240) DeprecationWarning: current URL string parser is deprecated, and will be removed in a future version. To use the new parser, pass option { useNewUrlParser: true } to MongoClient.connect.

The various methods by which you can connect to your mLab-hosted database are described here. We also provide some troubleshooting advice for issues you might encounter while trying to establish a connection.