[Validator] Fix a valid Date and Time constraint format

[adnen-chouibi]

I noticed that the date and time format specified at the start of the documentation is not correct, it should be Y-m-d for Date and H:i:s for Time instead of YYYY-MM-DD and HH:MM:SS

[xabbuh]

I think the previous formats were meant to be human-readable and not something that you could pass to date() or DateTimeInterface::format(). And I think not having the reader to know all the format options is a good thing. But can we somehow end up with something more clear than we have right now to avoid confusion?

[adnen-chouibi]

The Date constraint accepts only one format as mentioned in the example, $birthday it should be a string "Y-m-d" formatted value.
There is no other option to use it other than H:i:s for Time() and Y-m-d for Date().

[javiereguiluz]

In the code examples we mentioned that those were the only valid format ... but I agree that we could be explicit about this in the explanation too. I added some real example too in case any reader needs help with the PHP date formatting letters.

@adnen-chouibi thanks and congrats on your first Symfony Docs contribution 🎉