minor #11516 remove invalid array type (xabbuh)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes #11516).

Discussion
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remove invalid array type

This is not true before Symfony 4.3 (spotted while looking into how to document symfony/symfony#30429).

Commits
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4be9672 remove invalid array type

Author: javiereguiluz

reference/forms/types/options/group_by.rst.inc

@@ -1,7 +1,7 @@
 group_by
 ~~~~~~~~
 
-**type**: ``array``, ``callable`` or ``string`` **default**: ``null``
+**type**: ``string`` or ``callable`` **default**: ``null``
 
 You can group the ``<option>`` elements of a ``<select>`` into ``<optgroup>``
 by passing a multi-dimensional array to ``choices``. See the