docs(GeoJSON): update link to RFC page
docs(GeoJSON): update link to RFC page

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

Leaflet/Leaflet,CapeSepias/Leaflet,CapeSepias/Leaflet,Leaflet/Leaflet,Leaflet/Leaflet,CapeSepias/Leaflet,stefanocudini/Leaflet,stefanocudini/Leaflet,stefanocudini/Leaflet