minor #14855 Added missing Dotenv use (JohnstonCode)

This PR was merged into the 5.1 branch.

Discussion
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Added missing Dotenv use

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Commits
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7c0db22 Added missing Dotenv use

Author: javiereguiluz

migration.rst

@@ -239,6 +239,7 @@ could look something like this::
     use App\Kernel;
     use App\LegacyBridge;
     use Symfony\Component\Debug\Debug;
+    use Symfony\Component\Dotenv\Dotenv;
     use Symfony\Component\HttpFoundation\Request;
 
     require dirname(__DIR__).'/vendor/autoload.php';