3.13.4 release notes: correct a typo

Author: michaelklishin

release-notes/3.13.4.md

@@ -61,7 +61,7 @@ Release notes can be found on GitHub at [rabbitmq-server/release-notes](https://
 
 #### Enhancements
 
- * In some parallel cluster formation scenarios where definitions were [imported on node boot](),
+ * In some parallel cluster formation scenarios where definitions were [imported on node boot](https://www.rabbitmq.com/docs/definitions#import-on-boot),
    the virtual hosts created by the import can only be started on a subset of nodes. This is so
    because not all cluster peers are known at virtual host creation time.