fix link for Javascript promises

Author: nikomatsakis

posts/2019-11-07-Async-await-stable.md

@@ -71,6 +71,7 @@ new syntax:
 [hyper#1805]: https://github.com/hyperium/hyper/issues/1805
 [async-std]: https://async.rs/
 [wf]: https://www.arewewebyet.org/topics/frameworks/
+[JavaScript promises]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Using_promises
 
 ### Async-await: a quick primer