minor #9036 [Workflow] fix event name for completed event (dpfaffenbauer)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes #9036).

Discussion
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[Workflow] fix event name for completed event

Fixes the wrong event for the Workflow completed event.

Commits
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862962d [Workflow] fix event name for completed event

Author: xabbuh

workflow/usage.rst

@@ -246,7 +246,7 @@ order:
 
     The three events being dispatched are:
 
-    * ``workflow.transition``
+    * ``workflow.completed``
     * ``workflow.[workflow name].completed``
     * ``workflow.[workflow name].completed.[transition name]``