Update API.md to remove deprecated date.format syntax from an example

[WesTyler]

Removes v10.x deprecated Joi.date().format() syntax from API.md example, per #1063.

[AdriVanHoudt]

I don't think the example is still as clear as before with this change

[WesTyler]

@AdriVanHoudt you're right. That's what happens when I make assumptions. My bad.
I modified the example to utilize date().timestamp() and show the difference in behavior with and without .raw().

[AdriVanHoudt]

@WesTyler this makes it even better than before 🎉

[lock]

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