minor symfony#9071 Update choice.rst (jmangarret, weaverryan)

This PR was submitted for the 3.3 branch but it was merged into the 2.7 branch instead (closes symfony#9071).

Discussion
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Update choice.rst

FIX parametro atributo para añadir clase css

Commits
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f78b89e Adding the missing attr option to the ChoiceType
58a3043 Update choice.rst

Author: javiereguiluz

reference/forms/types/choice.rst

@@ -31,8 +31,9 @@ To use this field, you must specify *either* ``choices`` or ``choice_loader`` op
 | options     | - `empty_data`_                                                              |
 |             | - `error_bubbling`_                                                          |
 +-------------+------------------------------------------------------------------------------+
-| Inherited   | - `by_reference`_                                                            |
-| options     | - `data`_                                                                    |
+| Inherited   | - `attr`_                                                                    |
+| options     | - `by_reference`_                                                            |
+|             | - `data`_                                                                    |
 |             | - `disabled`_                                                                |
 |             | - `error_mapping`_                                                           |
 |             | - `inherit_data`_                                                            |
@@ -105,6 +106,7 @@ method::
         'choice_attr' => function($category, $key, $index) {
             return ['class' => 'category_'.strtolower($category->getName())];
         },
+        
         'group_by' => function($category, $key, $index) {
             // randomly assign things into 2 groups
             return rand(0, 1) == 1 ? 'Group A' : 'Group B';
@@ -327,6 +329,8 @@ Inherited Options
 These options inherit from the :doc:`form </reference/forms/types/form>`
 type:
 
+.. include:: /reference/forms/types/options/attr.rst.inc
+
 .. include:: /reference/forms/types/options/by_reference.rst.inc
 
 .. include:: /reference/forms/types/options/data.rst.inc