Update file convert-to-id-tokens.md

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What does this MR do and why?

Fixes the aud in the id_tokens tutorial for migrating from depricated (1.17) jwt to id_tokens for Vault. We came across this whilst making the required changes for 1.17.

Other parts of the docs use the vault address as the aud, not the gitlab address:

• https://docs.gitlab.com/ee/ci/secrets/id_token_authentication.html#configure-automatic-id-token-authentication
• https://docs.gitlab.com/ee/ci/examples/authenticating-with-hashicorp-vault/#example

MR acceptance checklist

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How to set up and validate locally

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