minor symfony#9404 Update upload_file.rst (TheSidSpears)

This PR was merged into the 4.0 branch.

Discussion
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Update upload_file.rst

$targetDirectory in service class, but $targetDir in services.yaml. Hm...

Commits
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c397ce7 Update upload_file.rst

Author: javiereguiluz

controller/upload_file.rst

@@ -271,7 +271,7 @@ Then, define a service for this class:
 
             App\Service\FileUploader:
                 arguments:
-                    $targetDir: '%brochures_directory%'
+                    $targetDirectory: '%brochures_directory%'
 
     .. code-block:: xml