Renamed: marker-icon@2x.png

In response to Issue:[1552](https://github.com/Leaflet/Leaflet/issues/1552).

I'm not sure if this file needs to be changed in a test somewhere or some other source svg or something... I just thought I would try and make it easy for you @mourner

Renamed `dist/images/marker-icon@2x.png` to `dist/images/marker-icon-at-2x.png`.