minor #8909 Update flex_recipes.rst (niumis)

This PR was merged into the 4.0 branch.

Discussion
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Update flex_recipes.rst

Missing 'use Doctrine\ORM\Mapping as ORM;'

Commits
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7866dcc Update flex_recipes.rst

Author: xabbuh

quick_tour/flex_recipes.rst

@@ -179,6 +179,7 @@ rich API for a ``product`` table? Create a ``Product`` entity and give it the
     // ...
 
     use ApiPlatform\Core\Annotation\ApiResource;
+    use Doctrine\ORM\Mapping as ORM;
 
     /**
      * @ORM\Entity()