Ok/Cancel buttons in Modal example are <spans> when rendered

[cvrebert]

1. Open the Modal docs page
2. Click the main "Open modal" button
3. View the DOM tree of the modal that appears
4. Observe that the Ok/Cancel buttons are:

<span class="button" zf-close="">Ok</span>
<span class="button" zf-close="">Cancel</span>

<span> aren't semantically appropriate for buttons. These should be <button>s or <a>s or at least have an ARIA role="button".

[gakimball]

Went ahead and converted them to anchors. As far as accessibility, it's something we'll have to wait to look at farther down the road when the framework is more solid.