3.12.0 release notes: add a missing link

Author: michaelklishin

release-notes/3.12.0.md

@@ -177,7 +177,7 @@ Release notes are kept under [rabbitmq-server/release-notes](https://github.com/
 
 This release includes all bug fixes shipped in the `3.11.x` series.
 
- * Classic mirrored queues that had a [length limit]() defined on them
+ * Classic mirrored queues that had a [length limit](https://rabbitmq.com/maxlength.html) defined on them
    handled the overflow of messages differently from the current elected leader,
    eventually causing exceptions and replica restarts.