Fix wrong property name for MultiUserSelectMenu initial value

[attepulkkinen]

Currently gives following error

{
  "ok": false,
  "error": "invalid_arguments",
  "warning": "missing_charset",
  "response_metadata": {
    "messages": [
      "[ERROR] failed to match all allowed schemas [json-pointer:\/view]",
      "[ERROR] invalid additional property: initial_user [json-pointer:\/view\/blocks\/1\/element]"
    ],
    "warnings": [
      "missing_charset"
    ]
  }
}

This is because correct property name for multi_users_select is initial_users instead of initial_user

[matthewnessworthy]

@attepulkkinen please can you take a look at this PR when you have a moment, it is currently blocking the usage of Kit::multiUserSelectMenu with initial users supplied

[maartenpaauw]

This seems to be a valid PR to me. According to the Slack Block Kit docs initial users should be set using the initial_users property: https://api.slack.com/reference/block-kit/block-elements#users_multi_select__fields

[maartenpaauw]

@attepulkkinen Sorry for the long wait, but it's released under version v2.1.1.