Update OpenStreetMap provider in gdal2tiles.py

I would like to suggest using the now preferred https://tile.openstreetmap.org URL instead of the current one (`{s}.`), see

openstreetmap/operations#737

Also, changes attribution URL to HTTPS link (use direct links).

opendm/tiles/gdal2tiles.py

@@ -2381,7 +2381,7 @@ def generate_leaflet(self):
 
         // Base layers
         //  .. OpenStreetMap
-        var osm = L.tileLayer('http://{s}.tile.osm.org/{z}/{x}/{y}.png', {attribution: '&copy; <a href="http://osm.org/copyright">OpenStreetMap</a> contributors'});
+        var osm = L.tileLayer('https://tile.openstreetmap.org/{z}/{x}/{y}.png', {attribution: '&copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> contributors'});
 
         //  .. CartoDB Positron
         var cartodb = L.tileLayer('http://{s}.basemaps.cartocdn.com/light_all/{z}/{x}/{y}.png', {attribution: '&copy; <a href="http://www.openstreetmap.org/copyright">OpenStreetMap</a> contributors, &copy; <a href="http://cartodb.com/attributions">CartoDB</a>'});