chore: use https

Author: hustcc

demo/index.html

@@ -43,9 +43,9 @@
   </div>
   <a class="github-url" href="https://github.com/hustcc/react-g2plot">react-g2plot</a>
 
-  <script src="https://unpkg.com/react@16/umd/react.production.min.js"></script>
-  <script src="https://unpkg.com/react-dom@16/umd/react-dom.production.min.js"></script>
-  <script src="http://unpkg.com/@antv/g2plot"></script>
+  <script src="//unpkg.com/react@16/umd/react.production.min.js"></script>
+  <script src="//unpkg.com/react-dom@16/umd/react-dom.production.min.js"></script>
+  <script src="//unpkg.com/@antv/g2plot"></script>
   <script src="./demo.min.js" type="application/javascript"></script>
 </body>
 </html>