minor #13027 Fix example with iterable (peterjaap)

This PR was submitted for the 5.0 branch but it was squashed and merged into the 4.4 branch instead (closes #13027).

Discussion
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Fix example with iterable

The old example throws this error; `PHP Fatal error:  Uncaught TypeError: Argument 1 passed to Symfony\Component\Console\Helper\ProgressBar::iterate() must be iterable, object given`

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Commits
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36b6e43 Fix example with iterable

Author: OskarStark

components/console/helpers/progressbar.rst

@@ -110,11 +110,17 @@ which starts, advances and finishes the progress bar automatically::
 
     $progressBar = new ProgressBar($output);
 
-    // $iterable can be for example an array ([1, 2, 3, ...]) or a generator
-    // $iterable = function () { yield 1; yield 2; ... };
+    // $iterable can be for example an array ([1, 2, 3, ...])
+    $iterable = [1, 2];
     foreach ($progressBar->iterate($iterable) as $value) {
         // ... do some work
     }
+    
+    // or a generator
+    function iterable() { yield 1; yield 2; ... };
+    foreach ($progressBar->iterate(iterable()) as $value) {
+        // ... do some work
+    }
 
 If ``$iterable = [1, 2]``, the previous code will output the following: