docs: update link to mplex spec

README.md

@@ -5,7 +5,7 @@
 [![](https://img.shields.io/badge/freenode-%23libp2p-yellow.svg?style=flat-square)](http://webchat.freenode.net/?channels=%23libp2p)
 [![Discourse posts](https://img.shields.io/discourse/https/discuss.libp2p.io/posts.svg)](https://discuss.libp2p.io)
 
-A super simple [stream muxing](https://docs.libp2p.io/concepts/stream-multiplexing/) library implementing [mplex](https://github.com/libp2p/mplex/).
+A super simple [stream muxing](https://docs.libp2p.io/concepts/stream-multiplexing/) library implementing [mplex](https://github.com/libp2p/specs/tree/master/mplex).
 
 ## Usage