minor #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 #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 | #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

Diff for: 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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