minor #3775 Fixed variable naming (peterrehm)

This PR was submitted for the 2.4 branch but it was merged into the 2.3 branch instead (closes #3775).

Discussion
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Fixed variable naming

Changed $app into $this since this is used in the other helpers as well and $app is not available inside a console command.

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | -

Commits
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66581a6 Fixed variable naming

components/console/helpers/tablehelper.rst

@@ -14,7 +14,7 @@ When building a console application it may be useful to display tabular data:
 To display a table, use the :class:`Symfony\\Component\\Console\\Helper\\TableHelper`,
 set headers, rows and render::
 
-    $table = $app->getHelperSet()->get('table');
+    $table = $this->getHelperSet()->get('table');
     $table
         ->setHeaders(array('ISBN', 'Title', 'Author'))
         ->setRows(array(