In many files, there are links to GTFS documentation.
It's used to on https://developers.google.com/ but it's now on https://gtfs.org/.
In some files, there are both links, keep only the gtfs.org link.
In some files, there are only the old developers.google.com links, replace with gtfs.org

Example links:

• https://developers.google.com/transit/gtfs/reference#agency_fields -> https://gtfs.org/schedule/reference/#agencytxt