Update schema id URL from NABSA to MobilityData repo

[richfab]

Fixes https://github.com/MobilityData/gbfs-json-schema/issues/101

This PR updates the schema id URLs from NABSA to MobilityData repo and fixes the anchors:

Before	After
"https://github.com/NABSA/gbfs/blob/v3.0-RC/gbfs.md#vehicle_typesjson-added-in-v21-rc",	"https://github.com/MobilityData/gbfs/blob/v3.0-RC/gbfs.md#vehicle_typesjson"-added-in-v21-rc,

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[richfab]

Good catch @davidgamez! I removed the validation of uniqueness from this PR and moved it to a separate PR: https://github.com/MobilityData/gbfs-validator/pull/159

[richfab]

@davidgamez This PR is ready for review. Thank you!