docs(GeoJSON): update link to RFC page

use the "new" RFC 7946 page (official GeoJSON specification) instead of the obsolete geojson.org website.

src/layer/GeoJSON.js

@@ -409,7 +409,7 @@ LayerGroup.include({
 // @namespace GeoJSON
 // @factory L.geoJSON(geojson?: Object, options?: GeoJSON options)
 // Creates a GeoJSON layer. Optionally accepts an object in
-// [GeoJSON format](http://geojson.org/geojson-spec.html) to display on the map
+// [GeoJSON format](https://tools.ietf.org/html/rfc7946) to display on the map
 // (you can alternatively add it later with `addData` method) and an `options` object.
 export function geoJSON(geojson, options) {
 	return new GeoJSON(geojson, options);