Fix typo in index.html

docs/index.html

@@ -25,7 +25,7 @@ title: Bootstrap
     <div class="row">
       <div class="col-sm-4">
         <img src="assets/img/sass-less.png" alt="Sass and Less support" class="img-responsive">
-        <h3>Preprocesors</h3>
+        <h3>Preprocessors</h3>
         <p>In addition to vanilla CSS, Bootstrap includes support for the two most popular CSS preprocessors, <a href="{{ page.base_url }}css#less">Less</a> and <a href="{{ page.base_url }}css#sass">Sass</a>.</p>
       </div>
       <div class="col-sm-4">