Update openlayer library links

Author: pramsey

examples/openlayers/openlayers-stereographic.html

@@ -25,8 +25,8 @@
   <title>OpenLayers Vector Tiles</title>
 
   <!-- CSS/JS for OpenLayers map  -->
-  <link rel="stylesheet" href="https://openlayers.org/en/v6.1.1/css/ol.css" type="text/css">
-  <script src="https://openlayers.org/en/v6.1.1/build/ol.js"></script>
+  <script src="https://cdn.jsdelivr.net/gh/openlayers/openlayers.github.io@master/en/v6.12.0/build/ol.js"></script>
+  <link rel="stylesheet" href="https://cdn.jsdelivr.net/gh/openlayers/openlayers.github.io@master/en/v6.12.0/css/ol.css">
 
   <style>
     body {