Fix the DAR upload example in the transaction failures guide

The example posts a multipart form to /api/validator/v0/admin/dars. The validator app does not serve that path (404, also with an admin token). Changed to the participant JSON Ledger API: POST /v2/dars with the raw DAR as the body.

Tested on LocalNet 0.9.2 (Canton 3.6.2): the raw body returns 200, the multipart form returns 400 INVALID_DAR, a non-admin token returns 403, no token returns 401. The json-ledger-api.localhost route to the participant on port 7575 is read from the Splice docker-compose nginx config.