fix: set the default mongo port to be the actual mongo port

[tpryan]

Currently pointing to 80 for historical reasons in the base tutorial. Let's change that.

[tpryan]

Under normal circumstances yes. But in a constrained sample where golang is not the main topic, I struggled with how to do this. I came to the conclusion that the less I ask of the user with this sample, the better. So hard code the port.