minor symfony#9561 Update formatter.rst (shapito27)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes symfony#9561).

Discussion
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Update formatter.rst

quote

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Commits
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4d00d47 Update formatter.rst

Author: xabbuh

logging/formatter.rst

@@ -69,7 +69,7 @@ configure your handler to use it:
                 'file' => array(
                     'type'      => 'stream',
                     'level'     => 'debug',
-                    'formatter' => JsonFormatter::class',
+                    'formatter' => JsonFormatter::class,
                 ),
             ),
         ));