minor symfony#11410 Add --dev flag to require symfony/debug-bundle (B-Galati)

This PR was merged into the 4.2 branch.

Discussion
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Add --dev flag to require symfony/debug-bundle

Commits
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417af04 Add --dev flag to require symfony/debug-bundle

Author: javiereguiluz

components/var_dumper.rst

@@ -21,7 +21,7 @@ Installation
 .. note::
 
     If using it inside a Symfony application, make sure that the DebugBundle has
-    been installed (or run ``composer require symfony/debug-bundle`` to install it).
+    been installed (or run ``composer require --dev symfony/debug-bundle`` to install it).
 
 .. _components-var-dumper-dump: