Remove "--unpack" option from setup.rst

The "--unpack" option seems to have disapeared ... some times ago

(i could not find the exact PR on the symfony/flex repository)

Author: smnandre

setup.rst

@@ -220,7 +220,7 @@ You won't see the ``symfony/debug-pack`` dependency in your ``composer.json``,
 as Flex automatically unpacks the pack. This means that it only adds the real
 packages as dependencies (e.g. you will see a new ``symfony/var-dumper`` in
 ``require-dev``). While it is not recommended, you can use the ``composer
-require --no-unpack ...`` option to disable unpacking.
+require --no-plugins ...`` option to disable Flex and prevent unpacking.
 
 .. _security-checker: