minor symfony#10728 Use the new HandlersLocator and initialise it correctly. (j92)

This PR was merged into the 4.2 branch.

Discussion
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Use the new HandlersLocator and initialise it correctly.

It looks like the HandlerLocator is replaced with the HandlersLocator in 4.2.

Here you can check the initialisation of the `HandlersLocator`: https://github.com/symfony/symfony/blob/4.2/src/Symfony/Component/Messenger/Tests/Handler/HandlersLocatorTest.php

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Commits
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c422cbc Use the new HandlersLocator and initialise it correctly.

Author: javiereguiluz

components/messenger.rst

@@ -83,12 +83,12 @@ Example::
 
     use App\Message\MyMessage;
     use Symfony\Component\Messenger\MessageBus;
-    use Symfony\Component\Messenger\Handler\Locator\HandlerLocator;
+    use Symfony\Component\Messenger\Handler\HandlersLocator;
     use Symfony\Component\Messenger\Middleware\HandleMessageMiddleware;
 
     $bus = new MessageBus([
-        new HandleMessageMiddleware(new HandlerLocator([
-            MyMessage::class => $handler,
+        new HandleMessageMiddleware(new HandlersLocator([
+            MyMessage::class => ['dummy' => $handler],
         ])),
     ]);