Update index.html

Updated summary wording.
 
Also, suggest that pictures fill their respective areas with words overlapping said pictures for the three periods (as seen in the header block). I recommend white text for pre-war, black text for wwIII, and white text for post-war. I will be working on these in the coming days. 

<hr class="featurette-divider">
<div class="pre-war-img"> 
    <div class='row'>
        <div class="container">
            <h1>History Emergent</h1>
            <h2>Help us write a story about the future of Human existence.</h2>
        </div>
    </div>
</div>

.pre-war-img {
    background: url(http://i.imgur.com/ywhjtHh.jpg) repeat;

.war-img {
    background: url(http://i.imgur.com/Z9wyL9A.jpg) repeat;

.post-war-img {
    background: url(http://i.imgur.com/2RUmvee.jpg) repeat;
Fixed typo