Dropdown: Sandbox code for Angular incorrect

[lizhuomeng71]

Info

If you go to the Dropdown page on our website, and in the Sandbox select Filterable. It sets the code as such:

<goa-form-item label="Basic dropdown">
  <goa-dropdown name="item" value="" [filterable]=true>
    <goa-dropdown-item value="red" label="Red"></goa-dropdown-item>
    <goa-dropdown-item value="green" label="Green"></goa-dropdown-item>
    <goa-dropdown-item value="blue" label="Blue"></goa-dropdown-item>
  </goa-dropdown>
</goa-form-item>

Either [filterable] needs to have [] removed, or the boolean value needs to be updated to be a string

Acceptance Criteria

1. Sandbox code for Dropdown is updated to be accurate when using the filterable property for Angular