docs(readme): replace npmcdn.com with unpkg.com

Author: npmcdn-to-unpkg-bot

README.md

@@ -113,9 +113,9 @@ Loading this module through a script tag will make the `IpfsBlockService` obj av
 the global namespace.
 
 ```html
-<script src="https://npmcdn.com/ipfs-block-service/dist/index.min.js"></script>
+<script src="https://unpkg.com/ipfs-block-service/dist/index.min.js"></script>
 <!-- OR -->
-<script src="https://npmcdn.com/ipfs-block-service/dist/index.js"></script>
+<script src="https://unpkg.com/ipfs-block-service/dist/index.js"></script>
 ```
 
 You can find the [API documentation here](API.md)