minor #15097 Update definitions.rst (mostefamed)

This PR was merged into the 4.4 branch.

Discussion
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Update definitions.rst

Rename the returned variable from setArgument method and the assigned variable to addArgument method

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Commits
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ad6075b Update definitions.rst

Author: javiereguiluz

service_container/definitions.rst

@@ -92,10 +92,10 @@ fetched from the container::
     
     // adds a new argument with the name of the argument
     // $argumentName = the name of the argument in the constructor
-    $argument = $definition->setArgument('$argumentName', $argumentValue);
+    $definition = $definition->setArgument('$argumentName', $argumentValue);
 
     // adds a new argument
-    $definition->addArgument($argument);
+    $definition->addArgument($argumentValue);
 
     // replaces argument on a specific index (0 = first argument)
     $definition->replaceArgument($index, $argument);