OSM attribution on Demo page

[cmonagle]

OSM requires attribution when using its tiles.

Looks like we could just plop this around line 135 here https://github.com/opennorth/represent-canada/blob/f35e46ee6945fc8eac6b5a021c59d8ab2adbaa57/finder/static/js/demo.js (Don't have dev env to test)

    attribution: 'Map data &copy; <a href="http://openstreetmap.org">OpenStreetMap</a> contributors, <a href="http://creativecommons.org/licenses/by-sa/2.0/">CC-BY-SA</a>, Imagery © <a href="http://mapbox.com">Mapbox</a>',

[jpmckinney]

Fixed, thanks!