Fix "date" in docs

[ssundarraj]

Docs on http://bendavis78.github.io/paper-date-picker/paper-date-picker/ read:

The selected date (YYYY-MM-DD)

That format seems to be invalid. I get an error:

Invalid date: 2018-02-02.

I'm unable to repro the console error on plnkr, but the date isn't set correctly.

http://plnkr.co/edit/APbI7Z61zgIEqDif1nIP?p=preview

[admwx7]

I'm unable to re-create your error as it's running fine for me in that plunkr, however I did notice that the date is off by 1 day, I'll open an issue for that one.