Mistake in Jumbotron docs

In the Jumbotron docs example, the foloowing code appears:
```
<p class="lead">
  <a class="btn btn-primary btn-lg" href="#" role="button">Learn more</a>
</p>
```


The 'lead' class here does nothing to the button and therefore should be omitted.
In addition, I think a div should be used instead of a paragraph because there is no real paragraph here, it just holds a button.
