bug #4010 [Console] Fixed documentation for ProgressBar (VasekPurchart)

This PR was merged into the 2.5 branch.

Discussion
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[Console] Fixed documentation for ProgressBar

This place is using the old ProgressHelper API and can be confusing when migrating.

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.5+
| Fixed tickets | no

Commits
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4bc00df Fixed documentation for ProgressBar

components/console/helpers/progressbar.rst

@@ -265,10 +265,11 @@ to display it can be customized::
     For performance reasons, be careful if you set the total number of steps
     to a high number. For example, if you're iterating over a large number of
     items, consider setting the redraw frequency to a higher value by calling
-    :method:`Symfony\\Component\\Console\\Helper\\ProgressHelper::setRedrawFrequency`,
+    :method:`Symfony\\Component\\Console\\Helper\\ProgressBar::setRedrawFrequency`,
     so it updates on only some iterations::
 
-        $progress->start($output, 50000);
+        $progress = new ProgressBar($output, 50000);
+        $progress->start();
 
         // update every 100 iterations
         $progress->setRedrawFrequency(100);