Add OpenStreetMap attribution to Leaflet example (#991)

* Add OpenStreetMap attribution to Leaflet example

* https

---------

Co-authored-by: Mike Bostock <[email protected]>

Author: elevenchars

docs/lib/leaflet.md

@@ -21,7 +21,9 @@ div.style = "height: 400px;";
 const map = L.map(div)
   .setView([51.505, -0.09], 13);
 
-L.tileLayer("https://tile.openstreetmap.org/{z}/{x}/{y}.png")
+L.tileLayer("https://tile.openstreetmap.org/{z}/{x}/{y}.png", {
+  attribution: '&copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a>'
+})
   .addTo(map);
 
 L.marker([51.5, -0.09])