minor symfony#4585 When explaining how to install dependencies for running unit tests, (carlosbuenosvinos)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes symfony#4585).

Discussion
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When explaining how to install dependencies for running unit tests,

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | symfony#4584

Commits
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512632e When explaining how to install dependencies for running unit tests, there are references to "--dev" composer parameter that is the current default value.

Author: wouterj

contributing/code/tests.rst

@@ -41,7 +41,7 @@ Step 2: Install vendors
 
 .. code-block:: bash
 
-    $ php composer.phar --dev install
+    $ php composer.phar install
 
 .. note::
 
@@ -56,14 +56,14 @@ Step 2: Install vendors
     .. code-block:: bash
 
         $ php installer
-        $ php composer.phar --dev install
+        $ php composer.phar install
 
 After installation, you can update the vendors to their latest version with
 the follow command:
 
 .. code-block:: bash
 
-    $ php composer.phar --dev update
+    $ php composer.phar update
 
 Running
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