我这切换符怎么显现成这样！

<a class="left carousel-control" href="#ad-carousel" data-slide="prev"><span

            class="glyphicon glyphicon-chevron-left"></span></a>

    <a class="right carousel-control" href="#ad-carousel" data-slide="next"><span

            class="glyphicon glyphicon-chevron-right"></span></a>