Use non-broken Sass link in docs; fixes part of #17314

[skip sauce]

docs/index.html

@@ -57,7 +57,7 @@ title: Bootstrap · The world's most popular mobile-first and responsive f
       <div class="col-sm-4 m-b-lg">
         <img src="assets/img/sass.png" alt="Sass support" class="img-responsive">
         <h4>Preprocessor</h4>
-        <p>Bootstrap ships with vanilla CSS, but its source code utilizes <a href="../css/#sass">Sass</a>, a popular CSS preprocessor. Quickly get started with precompiled CSS or build on the source.</p>
+        <p>Bootstrap ships with vanilla CSS, but its source code utilizes <a href="http://sass-lang.com/">Sass</a>, a popular CSS preprocessor. Quickly get started with precompiled CSS or build on the source.</p>
       </div>
       <div class="col-sm-4 m-b-lg">
         <img src="assets/img/devices.png" alt="Responsive across devices" class="img-responsive">