Use `yyyy-mm-dd` format instead of `dd-mm-yyyy` in example.

[mjzac]

L3 milestone uses yyyy-mm-dd https://github.com/pupilfirst/wd201-scripts/blob/v2/nodejs-deep-dive/milestone/README.md

and the lesson just before that uses dd-mm-yyyy https://github.com/pupilfirst/wd201-scripts/blob/v2/nodejs-deep-dive/todo-using-closure/README.md

Need to be consistent in the usage and stick to a single format.