minor #17855 import function env to avoid fatal error if copying docs (PhilETaylor)

This PR was squashed before being merged into the 5.4 branch.

Discussion
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import function env to avoid fatal error if copying docs

If you copy and past the example code, or dont realise then you get a fatal error message as `env` is a function not within the global scope.

<img width="1139" alt="ScreenShot-2023-02-02-21 25 27" src="https://user-images.githubusercontent.com/400092/216453124-166c7331-a0e6-4e1c-aff8-3aa6e4dfcc48.png">

This change in the docs just imports the function like phpStorm tells me to

```
use function Symfony\Component\DependencyInjection\Loader\Configurator\env;
```

Commits
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32dfcbd import function env to avoid fatal error if copying docs

Author: javiereguiluz

mailer.rst

@@ -54,6 +54,7 @@ over SMTP by configuring the DSN in your ``.env`` file (the ``user``,
     .. code-block:: php
 
         // config/packages/mailer.php
+        use function Symfony\Component\DependencyInjection\Loader\Configurator\env;
         use Symfony\Component\DependencyInjection\Loader\Configurator\ContainerConfigurator;
 
         return static function (ContainerConfigurator $containerConfigurator): void {
@@ -1193,6 +1194,7 @@ This can be configured by replacing the ``dsn`` configuration entry with a
     .. code-block:: php
 
         // config/packages/mailer.php
+        use function Symfony\Component\DependencyInjection\Loader\Configurator\env;
         use Symfony\Config\FrameworkConfig;
 
         return static function (FrameworkConfig $framework) {