Add mix pleroma.user unsubscribe to mix doc

lib/mix/tasks/pleroma/user.ex

@@ -31,6 +31,10 @@ defmodule Mix.Tasks.Pleroma.User do
 
       mix pleroma.user toggle_activated NICKNAME
   
+  ## Unsubscribe local users from user's account and deactivate it
+     
+      mix pleroma.user unsubscribe NICKNAME
+
   ## Create a password reset link.
 
       mix pleroma.user reset_password NICKNAME