Harmonize ELSER model ID

[kosabogi]

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Related issue: https://github.com/elastic/search-docs-team/issues/258

Changes

Deprecation message after running the PUT _inference/sparse_embedding/my-elser-model call:

PUT _inference/sparse_embedding/my-elser-model

#! Putting elasticsearch service inference endpoints (including elser service) without a model_id field is deprecated and will be removed in a future release. Please specify a model_id field.
#! The [elser] service is deprecated and will be removed in a future release. Use the [elasticsearch] service instead, with [model_id] set to [.elser_model_2_linux-x86_64] in the [service_settings]

I've updated the example to reflect this change by:

• Replacing "service": "elser" with "service": "elasticsearch".
• Adding `"model_id"

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[dlockli]

LGTM @kosabogi!