using myDiv as name for graph container for geojson example

Author: Joseph Damiba

_posts/plotly_js/maps/fill-area-on-mapbox/2019-09-02-geojson_layers.html

@@ -56,4 +56,4 @@
 
 var config = {mapboxAccessToken: "your access token"};
 
-Plotly.newPlot("graph", data, layout, config);
+Plotly.newPlot("myDiv", data, layout, config);