minor symfony#10960 public access modifier for const (iliyaZelenko)

This PR was merged into the 4.2 branch.

Discussion
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public access modifier for const

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Commits
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bf3801e Update event_dispatcher.rst

Author: javiereguiluz

components/event_dispatcher.rst

@@ -262,7 +262,7 @@ order. Start by creating this custom event class and documenting it::
      */
     class OrderPlacedEvent extends Event
     {
-        const NAME = 'order.placed';
+        public const NAME = 'order.placed';
 
         protected $order;