minor #9968 Install component for dev environment (seferov)

This PR was merged into the 2.8 branch.

Discussion
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Install component for dev environment

I think VarDumper is meant to be installed for development environment. So it is better to install it under `require-dev` most of the time

Commits
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e2e4765 Install component for dev environment

Author: javiereguiluz

components/var_dumper.rst

@@ -14,7 +14,7 @@ Installation
 
 .. code-block:: terminal
 
-    $ composer require symfony/var-dumper
+    $ composer require symfony/var-dumper --dev
 
 Alternatively, you can clone the `<https://github.com/symfony/var-dumper>`_ repository.