Fix Github icon in docs

docs/_includes/header.html

@@ -17,10 +17,8 @@
         <a class="page-link" href="{{ site.github.url }}/guides/">Guides</a>
         <a class="page-link" href="{{ site.github.url }}/faq/">FAQ</a>
         <a class="page-link" href="https://github.com/box/spout">
-            GitHub
-            <svg width="12" height="12">
-                <image xlink:href="/images/external-link.svg" src="/images/external-link.png" width="12" height="12"/>
-            </svg>
+            <span class="icon icon--github">{% include icon-github.svg %}</span>
+            <span class="username">GitHub</span>
         </a>
         {% if site.algolia.enabled %}
         <span class="site-search">