Fix polyfill.io link (#16)

csabapalfi · fpapado · commit a1c22355df9e · 2018-09-25T10:37:06.000+03:00
diff --git a/README.md b/README.md
@@ -395,7 +395,7 @@ And import it at your app's entry point:
 import "intersection-observer";
 ```
 
-[Polyfill.io is an alternative method of distributing the polyfill](polyfill.io) if you wish.
+[Polyfill.io is an alternative method of distributing the polyfill](https://polyfill.io) if you wish.
 
 #### About the polyfill
 
