gtfs-api OpenAPI spec: set default value for `limit` parameter

[derhuerst]

By default, when using gtfs-api OpenAPI spec via the gtfs-swagger-ui, it freezes when fetching many results (PostgREST has a default limit of 100k or 1m).

This is currently blocked by PostgREST/postgrest#2871 as long as we don't manually post-process the OpenAPI spec.