Use more permanent URL

gtfs/spec/en/reference.md

@@ -1,4 +1,4 @@
-## General Transit Feed Specification Reference
+![image](https://github.com/leonardehrenfried/transit/assets/151346/22a1f94c-34b0-452b-a3d0-5cb296d1a0ba)## General Transit Feed Specification Reference
 
 **Revised May 22, 2024. See [Revision History](https://gtfs.org/schedule/process/#revision-history) for more details.**
 
@@ -747,7 +747,7 @@ File: **Optional**
 Defines zones where riders can request either pickup or drop off by on-demand services. These zones are represented as GeoJSON polygons.
 
 - This file uses a subset of the GeoJSON format, described in [RFC 7946](https://tools.ietf.org/html/rfc7946).
-- Each polygon must be valid by the definition of the [OpenGis Simple Features Specification, section 6.1.11](https://portal.ogc.org/files/?artifact_id=25355).
+- Each polygon must be valid by the definition of the [OpenGis Simple Features Specification, section 6.1.11](http://www.opengis.net/doc/is/sfa/1.2.1).
 - The `locations.geojson` file must contain a `FeatureCollection`.
 - A `FeatureCollection` defines various stop locations where riders may request pickup or drop off.
 - Every GeoJSON `Feature` must have an `id`. The `id` must be unique across all `stops.stop_id`, locations.geojson `id`, and `location_group_id` values.