minor #15398 Complete App\Handler\HandlerCollection namespace (BrnvrlUoeey)

This PR was submitted for the 5.2 branch but it was merged into the 4.4 branch instead.

Discussion
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Complete App\Handler\HandlerCollection namespace

Convert App\HandlerCollection to App\Handler\HandlerCollection in code snippet in accordance with text description above.

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Commits
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b0d93f3 Complete App\Handler\HandlerCollection namespace

Author: javiereguiluz

service_container/service_subscribers_locators.rst

@@ -494,7 +494,7 @@ of the ``key`` tag attribute (as defined in the ``index_by`` locator option):
                 tags:
                     - { name: 'app.handler', key: 'handler_two' }
 
-            App\HandlerCollection:
+            App\Handler\HandlerCollection:
                 # inject all services tagged with app.handler as first argument
                 arguments: [!tagged_locator { tag: 'app.handler', index_by: 'key' }]