bug #6787 Fix reference to output object (micheal)

This PR was submitted for the 3.1 branch but it was merged into the 2.7 branch instead (closes #6787).

Discussion
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Fix reference to output object

`$this` does not have a `writeln` method on it, but `$output` does.

Commits
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4deb3f0 Fix reference to output object

console.rst

@@ -131,7 +131,7 @@ Use input options or arguments to pass information to the command::
         ]);
 
         // retrieve the argument value using getArgument()
-        $this->writeln('Username: '.$input->getArgument('username'));
+        $output->writeln('Username: '.$input->getArgument('username'));
     }
 
 Now, you can pass the username to the command: