minor #5552 rename $input to $greetInput (Xosofox)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #5552).

Discussion
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rename $input to $greetInput

Not a good practice to set/abuse a variable that was passed as an argument

Commits
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53b4a3d rename $input to $greetInput

components/console/introduction.rst

@@ -469,8 +469,8 @@ Calling a command from another one is straightforward::
             '--yell'  => true,
         );
 
-        $input = new ArrayInput($arguments);
-        $returnCode = $command->run($input, $output);
+        $greetInput = new ArrayInput($arguments);
+        $returnCode = $command->run($greetInput, $output);
 
         // ...
     }