Fix link to the user mailing list

Author: garydgregory

README.md

@@ -57,7 +57,7 @@ Documentation
 
 More information can be found on the [Apache Commons CLI homepage](https://commons.apache.org/proper/commons-cli).
 The [Javadoc](https://commons.apache.org/proper/commons-cli/apidocs) can be browsed.
-Questions related to the usage of Apache Commons CLI should be posted to the [user mailing list][ml].
+Questions related to the usage of Apache Commons CLI should be posted to the [user mailing list](https://commons.apache.org/mail-lists.html).
 
 Getting the latest release
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