Fix typo "definitly" -> "definitely" (#38)

Author: sibeitokgong

docs/index.html

@@ -172,7 +172,7 @@ <h3>Math demo:</h3>
                 <p data-height="512" data-theme-id="0" data-slug-hash="yXVxYX" data-default-tab="js,result" data-user="arguiot" data-embed-version="2" data-pen-title="yXVxYX" class="codepen">See the Pen <a href="https://codepen.io/arguiot/pen/yXVxYX/">DisplayJS - Example 3</a> by Arthur  Guiot (<a href="https://codepen.io/arguiot">@arguiot</a>) on <a href="https://codepen.io">CodePen</a>.</p>
                 <script async src="https://production-assets.codepen.io/assets/embed/ei.js"></script>
             </div>
-            <h2>And, DisplayJS is more than that! If you already like this, you can definitly read the <a href="https://github.com/arguiot/DisplayJS/wiki">docs</a>!</h2>
+            <h2>And, DisplayJS is more than that! If you already like this, you can definitely read the <a href="https://github.com/arguiot/DisplayJS/wiki">docs</a>!</h2>
         </section>
         <!-- #main -->
     </div>