minor #5473 --dev is default and causes a warning (DQNEO)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #5473).

Discussion
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--dev is default and causes a warning

--dev causes a warning:
```
$ composer --dev update
You are using the deprecated option "dev". Dev packages are installed by default now.
```

Commits
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ce518db --dev is default and causes a warning

Author: xabbuh

contributing/code/tests.rst

@@ -45,7 +45,7 @@ the follow command:
 
 .. code-block:: bash
 
-    $ composer --dev update
+    $ composer update
 
 Running
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