JSON attribute should be `initial_options` instead of `initial_option`

[caalberts]

@CGA1123 Either this is a bug or Slack actually allows singular version of the attribute.

• MultiExternalSelect

  slack-ruby-block-kit/lib/slack/block_kit/element/multi_external_select.rb

  Line 52 in 371f2f2

  initial_option: @initial_option&.as_json,

  • https://api.slack.com/reference/block-kit/block-elements#external_multi_select__fields
• MultiStaticSelect

  slack-ruby-block-kit/lib/slack/block_kit/element/multi_static_select.rb

  Line 80 in 371f2f2

  initial_option: @initial_option&.as_json,

  • https://api.slack.com/reference/block-kit/block-elements#static_multi_select__fields

[anvox]

I didn't test it properly. It should be initial_options, initial_option is for single choice. You could verify it here

Could you help to open a PR to fix it? @caalberts
if you're busy, I'll back to fix it tomorrow. Thanks :)

[CGA1123]

Hey @caalberts

This is correct. MultiConversationsSelect, MultiChannelsSelect, and MultiUsersSelect are also affected by being singular rather than plural.