minor #14473 Fix typo (ttskch)

This PR was merged into the 3.4 branch.

Discussion
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Fix typo

`0` (Zero) is correct instead of `O` (Alphabet O).

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Commits
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38da641 Fix typo

Author: wouterj

testing.rst

@@ -757,7 +757,7 @@ their type::
     $form['photo']->upload('/path/to/lucas.jpg');
 
     // In the case of a multiple file upload
-    $form['my_form[field][O]']->upload('/path/to/lucas.jpg');
+    $form['my_form[field][0]']->upload('/path/to/lucas.jpg');
     $form['my_form[field][1]']->upload('/path/to/lisa.jpg');
 
 .. tip::