IE10 Mobile viewport bug workaround: document.querySelector('head') => document.head

Per http://caniuse.com/#feat=documenthead , document.head is supported in IE Mobile 10+,
so slightly simplify the JS accordingly.

[skip sauce]

docs/assets/js/ie10-viewport-bug-workaround.js

@@ -17,7 +17,7 @@
         '@-ms-viewport{width:auto!important}'
       )
     )
-    document.querySelector('head').appendChild(msViewportStyle)
+    document.head.appendChild(msViewportStyle)
   }
 
 })();

docs/getting-started/browsers-devices.md

@@ -181,7 +181,7 @@ if (navigator.userAgent.match(/IEMobile\/10\.0/)) {
       '@-ms-viewport{width:auto!important}'
     )
   )
-  document.querySelector('head').appendChild(msViewportStyle)
+  document.head.appendChild(msViewportStyle)
 }
 {% endhighlight %}