Fix site variable in header.html

Author: jeroendesloovere

_includes/header.html

@@ -26,6 +26,6 @@
     m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
     })(window,document,'script','//www.google-analytics.com/analytics.js','ga');
 
-    ga('create', '{% site.google_analytics %}', 'auto');
+    ga('create', '{{ site.google_analytics }}', 'auto');
     ga('send', 'pageview');
 </script>