Docs: improve cards image overlays markup

[julien-deramond]

All cards examples have the same structure regarding the content:

  <div class="card-body">
    <h5 class="card-title">Card title</h5>
    <p class="card-text">This is a wider card with supporting text below as a natural lead-in to additional content. This content is a little bit longer.</p>
    <p class="card-text"><small class="text-muted">Last updated 3 mins ago</small></p>
  </div>

The one in Image overlays doesn't contain the <small class="text-muted">...</small>.
Since .text-muted makes it non-accessible, I chose to only add the <small> HTML element.

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