Update README.md

Author: f

README.md

@@ -71,7 +71,7 @@ You can download `graphql.js` directly, or you can use **Bower** or **NPM**.
 #### CDN
 
 ```html
-<script src="https://cdn.jsdelivr.net/npm/graphql.js@0.6.6/graphql.min.js"></script>
+<script src="//cdn.jsdelivr.net/npm/graphql.js@0.6.6/graphql.min.js"></script>
 ```
 
 #### Download for Browser