minor #19646 Update contributing/code/tests.rst page (AurelienPillevesse)

This PR was squashed before being merged into the 7.0 branch.

Discussion
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Update contributing/code/tests.rst page

For 7.0 version

Commits
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075c29f Update contributing/code/tests.rst page

Author: javiereguiluz

contributing/code/tests.rst

@@ -32,7 +32,7 @@ tests, such as Doctrine, Twig and Monolog. To do so,
 
     .. code-block:: terminal
 
-        $ COMPOSER_ROOT_VERSION=6.4.x-dev composer update
+        $ COMPOSER_ROOT_VERSION=7.0.x-dev composer update
 
 .. _running: