Update endpoint.update signature

[Wauplin]

Two parameters are missing compared to the update_inference_endpoint method: scale_to_zero_timeout and custom_image. This PR fixes it.
Reported by @MoritzLaurer in #2472 (comment).

[HuggingFaceDocBuilderDev]

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[MoritzLaurer]

LGTM, thanks!
(I imagine that the token parameter is not necessary because the token is already in the endpoint object?)

[Wauplin]

Thanks for the review!

(I imagine that the token parameter is not necessary because the token is already in the endpoint object?)

Yes exactly, it's already passed as token=self._token.