Fixed a typo in the documentation for the page-header class

Created by: coryalder

Offending line can be found here: http://twitter.github.com/bootstrap/components.html#typography

<div class="page-haeder">
    <h1>Example page header</h1>
</div>

Should be:

<div class="page-header">
    <h1>Example page header</h1>
</div>