minor symfony#6449 [Form] fixed ChoiceType example in CollectionType (HeahDude)

This PR was merged into the 2.7 branch.

Discussion
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[Form] fixed ChoiceType example in CollectionType

| Q | A |
|---|-----|
| Branch | 2.7+ |
| Doc fix | yes |

Uses new ChoiceType api.

Commits
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80c26c1 [Form] fixed ChoiceType example in CollectionType

Author: weaverryan

reference/forms/types/collection.rst

@@ -289,11 +289,12 @@ type::
         'type'   => 'choice',
         'options'  => array(
             'choices'  => array(
-                'nashville' => 'Nashville',
-                'paris'     => 'Paris',
-                'berlin'    => 'Berlin',
-                'london'    => 'London',
+                'Nashville' => 'nashville',
+                'Paris'     => 'paris',
+                'Berlin'    => 'berlin',
+                'London'    => 'london',
             ),
+            'choices_as_values' => true,
         ),
     ));