Update setup.rst

Author: nayzo

setup.rst

@@ -112,7 +112,7 @@ vulnerability. Run this command to install it in your application:
 .. code-block:: terminal
 
     $ cd my-project/
-    $ composer require sec-checker
+    $ composer require sec-checker --dev
 
 From now on, this utility will be run automatically whenever you install or
 update any dependency in the application. If a dependency contains a vulnerability,