minor #3842 Update dialoghelper.rst (bijsterdee)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #3842).

Discussion
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Update dialoghelper.rst

Consistent use of $this->getHelperSet()

Commits
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2734511 Update dialoghelper.rst

components/console/helpers/dialoghelper.rst

@@ -192,7 +192,7 @@ Instead, you can use the
 method, which makes sure that the user can only enter a valid string
 from a predefined list::
 
-    $dialog = $app->getHelperSet()->get('dialog');
+    $dialog = $this->getHelperSet()->get('dialog');
     $colors = array('red', 'blue', 'yellow');
 
     $color = $dialog->select(