未对齐的Container Div-- Twitter Bootstrap

Question

我又做错了什么？ Placeholder列应与容器div对齐，但不是。这是代码

<div class="container">
<!--Content Area-->
<div id="content" class="row-fluid">

    <!--Main Content Area-->
    <div class="span6" id="main">
        <h2>PLACEHOLDER</h2>
        <div class="hero-unit">
            <h1>Marketing stuff!</h1>
            <br />
            <p>Cras justo odio, dapibus ac facilisis in, egestas eget quam. <br />Fusce dapibus, tellus ac cursus commodo, tortor mauris<br /> condimentum nibh, ut fermentum massa justo sit amet risus.</p>
            <br />
            <br />

            <a href="#" class="btn btn-large btn-success">Get Started</a>
        </div>
        <p>Cras justo odio, dapibus ac facilisis in, egestas eget quam. Fusce dapibus, tellus ac cursus commodo, tortor mauris condimentum nibh, ut fermentum massa justo sit amet risus.</p>
    </div>

    <!--Right Sidebar-->
    <div class="span3" id="right-sidebar">
    </div>

</div>