add class to b-dropdown-item code example

Example: bootstrap 4 vue dropdown

<div>
  <b-dropdown text="Block Level Dropdown" block variant="primary" class="m-2">
    <b-dropdown-item href="#">Action</b-dropdown-item>
    <b-dropdown-item href="#">Another action</b-dropdown-item>
    <b-dropdown-item href="#">Something else here</b-dropdown-item>
  </b-dropdown>

  <b-dropdown
    text="Block Level Split Dropdown"
    block
    split
    split-variant="outline-primary"
    variant="primary"
    class="m-2"
  >
    <b-dropdown-item href="#">Action</b-dropdown-item>
    <b-dropdown-item href="#">Another action</b-dropdown-item>
    <b-dropdown-item href="#">Something else here...</b-dropdown-item>
  </b-dropdown>
</div>