docs: add PR link to 3.1.0 release notes

Author: fgmacedo

docs/releases/3.1.0.md

@@ -10,5 +10,6 @@
   transition to `Event()` (e.g., `Event(t1, t2)`) now raises {ref}`InvalidDefinition` with a
   clear message suggesting the `|` operator. Previously, the second argument was silently
   interpreted as the event `id`, leaving the extra transitions eventless (auto-firing).
+  [#588](https://github.com/fgmacedo/python-statemachine/pull/588).
 
 ## Misc in 3.1.0