Update screens.json

Hello Lea,

I just wanted to add the specifications of the *Moto G* by *Motorola*.

But before I've written down the JSON, I checked the window-object of my mobile firefox/chrome and it tells different attributes to the physical ones:
So the dpi is `192`, while the physical ppi is `326`.

If you want to add the `dpi` of the window-object to your list, I added it to the JSON.