docs: POST /v2/model shows incorrect JSON field in example

[BAMF0]

Remodeling using the /v2/model endpoint requires sending a JSON body like this:

{
  "new-model": <raw_model_assertion_text>
}

However the API docs show:

{
  "assertion": <raw_model_assertion_text>
}

which throws the error cannot decode new model assertion: assertion content/signature separator not found.

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*Reported from: https://snapcraft.io/docs/reference/development/snapd-rest-api/

[Rnfudge02]

Thank you for the issue and pull request, the changes have been brought into snapd via the following PR!

canonical/snapd#17199