fix: docstring in endpoint_service.py

[fabiogra]

The docstring in the class UndeployModelOperator regarding the endpoint_id is not correct. This parameter accept only the id of the endpoint and not the path as written projects/{project}/locations/{location}/endpoints/{endpoint}

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^ Add meaningful description above
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[boring-cyborg]

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[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.