Fix GitHub format

Github -> GitHub

Author: Ikko Ashimine

docs/_includes/header.html

@@ -12,7 +12,7 @@
                     <i class="fa fa-download"></i> Download
                 </a>
                 <a class="item" href="https://github.com/usablica/intro.js/" target="_blank">
-                    <i class="fa fa-github"></i> Github
+                    <i class="fa fa-github"></i> GitHub
                 </a>
                 <a class="item" href="{{site.baseurl}}/support" target="_blank">
                     <i class="fa fa-support"></i> Support