Fix link to the user mailing list

Author: garydgregory

README.md

@@ -60,7 +60,7 @@ Documentation
 
 More information can be found on the [Apache Commons Codec homepage](https://commons.apache.org/proper/commons-codec).
 The [Javadoc](https://commons.apache.org/proper/commons-codec/apidocs) can be browsed.
-Questions related to the usage of Apache Commons Codec should be posted to the [user mailing list][ml].
+Questions related to the usage of Apache Commons Codec should be posted to the [user mailing list](https://commons.apache.org/mail-lists.html).
 
 Where can I get the latest release?
 -----------------------------------