minor symfony#2470 [Autocomplete] Fix invalid createQueryBuilder code example (tacman, smnandre)

This PR was merged into the 2.x branch.

Discussion
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[Autocomplete] Fix invalid createQueryBuilder code example

replace with creating it from the entity repository

Commits
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c028c19 Update index.rst
27e539b replace undefined $qb

Author: smnandre

src/Autocomplete/doc/index.rst

@@ -315,8 +315,9 @@ the consequent renders, the default ``query_builder`` will be used::
             $builder
                 ->add('food', FoodAutocompleteField::class, [
                     'query_builder' => function (EntityRepository $er) {
-                        return $er->createQueryBuilder('o')
-                            ->andWhere($qb->expr()->notIn('o.id', [$currentFoodId]));
+                        $qb = $er->createQueryBuilder('o');
+
+                        return $qb->andWhere($qb->expr()->notIn('o.id', [$currentFoodId]));
                     };
                 ])
             ;