minor #10400 [Dotenv] Advise using the Composer --dev option (ogizanagi)

This PR was merged into the 3.4 branch.

Discussion
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[Dotenv] Advise using the Composer --dev option

As we don't recommend using it on production.

Also relates to symfony/recipes#466 (comment)

Commits
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2e13c8b [Dotenv] Advise using the Composer --dev option

Author: javiereguiluz

components/dotenv.rst

@@ -16,7 +16,7 @@ Installation
 
 .. code-block:: terminal
 
-    $ composer require symfony/dotenv
+    $ composer require --dev symfony/dotenv
 
 Alternatively, you can clone the `<https://github.com/symfony/dotenv>`_ repository.