Fix curl commands on API page model_registry.md

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Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/509818

This MR is related to the open issue: #509818 titled Assess curl commands on API pages

There are two occurrences of the old curl command syntax, both occurrences are going to be updated by merge request.

The API page has been updated based on guidance for curl commands in the REST API pages (https://docs.gitlab.com/ee/development/documentation/restful_api_styleguide.html#curl-commands)