Google-map-marker: Adding more doc for icon property

[Nicolas Delperdange]

The doc to set an icon for marker is not clear. (Multiple issues seen with this error)

The description 'Image URL for the marker icon.' makes think that we just need to put an URL as a string but icon is an object, so the value will be null.

Maybe put an example like :

<google-map-marker icon='{"url":"https://developers.google.com/maps/documentation/javascript/examples/full/images/beachflag.png"}'></google-map-marker>

Thanks !