Fix bootstrap CDN link for HTTPS users

Author: empyrical

doc/pages/_layout.jade

@@ -5,7 +5,7 @@ html(lang=en)
  meta(http-equiv="X-UA-Compatible" content="IE=edge")
  meta(name="viewport" content="width=device-width, initial-scale=1")
  title react-native-renderer
- link(rel="stylesheet" href="http://netdna.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css")
+ link(rel="stylesheet" href="//netdna.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css")
  style(type='text/css').
  body {
  padding-top: 50px;