minor symfony#15902 [Form] Add missing "extends AbstractType" (sebpacz)

This PR was merged into the 4.4 branch.

Discussion
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[Form] Add missing "extends AbstractType"

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Commits
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6d538e5 [Form] Add missing "extends AbstractType"

Author: javiereguiluz

form/form_themes.rst

@@ -337,7 +337,7 @@ following complex example where a ``TaskManagerType`` has a collection of
         }
     }
 
-    class TaskType
+    class TaskType extends AbstractType
     {
         public function buildForm(FormBuilderInterface $builder, array $options = []): void
         {