3.13.4 release notes: a typo

Author: michaelklishin

release-notes/3.13.4.md

@@ -29,7 +29,7 @@ Release notes can be found on GitHub at [rabbitmq-server/release-notes](https://
 
 ### Core Broker
 
-#### Bug Fix
+#### Bug Fixes
 
  * A rolling upgrade from `3.12.14` to `3.13.x` could run into an exception.